Britain was braced for further terrorist attacks following the Manchester atrocity as Theresa May said the threat level was being raised to its highest possible rating.

In a live televised statement from Downing Street, the Prime Minister said the terror threat level would be increased to "critical", meaning an attack was expected imminently as it was possible a "wider group of individuals" could have been involved in the Manchester Arena attack rather than just suicide bomber Salman Abedi.

In a sign of the increased threat, the military could be deployed to support armed police officers, Mrs May said.

Monday night's attack at a concert by US pop star Ariana Grande left 22 people dead, including an eight-year-old girl, and dozens injured.

Speaking at Number 10, Mrs May said: "The spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists.

"That is why the terrorists will never win and we will prevail."

Mrs May will chair a meeting of the Government's emergency Cobra committee at 9.30am on Wednesday, Downing Street has announced.

The suicide bomber has been named as Salman Abedi, a 22-year-old man believed to have been born in Manchester and of Libyan descent.

Abedi studied business at Salford University but dropped out before completing his degree.

He is thought to have attended the Manchester Islamic Centre, also known as Didsbury Mosque, along with his parents and siblings.

A family friend, who asked not to be named, described him as "normal" and said they were known to the Libyan community in the city.

It comes after an eight-year-old girl was among the 22 people killed by a suicide bomber after an Ariana Grande concert at the Manchester Arena.

Saffie Roussos, a "beautiful little girl", was killed in the attack which ripped through the venue on Monday night.

In a statement released through Lancashire County Council, Tarleton Community Primary School headteacher Chris Upton said: "News of Saffie's death in this appalling attack has come as a tremendous shock to all of us and I would like to send our deepest condolences to all of her family and friends.

"The thought that anyone could go out to a concert and not come home is heartbreaking."

It continued: "Saffie was simply a beautiful little girl in every aspect of the word. She was loved by everyone and her warmth and kindness will be remembered fondly. Saffie was quiet and unassuming with a creative flair.

"Our focus is now on helping pupils and staff cope with this shocking news and we have called in specialist support from Lancashire County Council to help us do that.

"We are a tight-knit school and wider community and will give each other the support that we need at this difficult time."

The schoolgirl had been at the concert with her mother, Lisa Roussos, and sister, Ashlee Bromwich, in her 20s, from Leyland, Lancashire.

They are both now in separate hospitals being treated for injuries, friends said.

Before confirmation of Saffie's death, they posted a Facebook appeal for information about her whereabouts.

Earlier in the day, the first victim of the Manchester bomb attack at the city's arena was named by her college as Georgina Callander.

Runshaw College in Leyland, Lancashire, spoke of its "enormous sadness" after hearing that the student died as a result of the blast at the Ariana Grande concert.

A spokesman said in a statement the young girl was on the second year of her health and social care course, and was a former student at Bishop Rawstorne Church of England Academy.

"Our deepest sympathies, thoughts and prayers go out to all of Georgina's family, friends, and all of those affected by this loss," the college said.

"We are offering all available support possible at this tragic time, including counselling with our dedicated student support team."

Bishop Rawstorne Church of England Academy confirmed that Georgina died as a result of the injuries she sustained.

In a statement the school said: "Georgina was a lovely young student who was very popular with her peers and the staff and always made the most of the opportunities she had at the school.

"Our thoughts and prayers go out to Georgina's family at this terrible time, and we think especially of her brothers Harry and Daniel who were also former students of the school.

"All of our students will gather together today for a time of prayer and reflection and to give thanks for the life of Georgina."

Witnesses reported hearing a "huge bang" at the venue during a sold-out Ariana Grande gig on Monday evening.

Some 59 people were also injured when the blast caused by an improvised explosive device carried by the attacker detonated at the Manchester Arena.

Announcing that the death toll had risen, Greater Manchester Police chief constable Ian Hopkins said: "What I can confirm is that there are children among the deceased."

He said: "This has been the most horrific incident we have had to face in Greater Manchester and one that we all hoped we would never see.

"Families and many young people were out to enjoy a concert at the Manchester Arena and have lost their lives.

"Our thoughts are with those 22 victims that we now know have died, the 59 people who have been injured and their loved ones.

"We continue to do all we can to support them."

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Police have given an emergency number for those who are concerned about loved ones or anyone who may have been in the area - 0161 856 9400.

Casualties were being treated at six hospitals across Greater Manchester.

Meanwhile, commuters waking up this morning have been told to check latest travel information after trains to Manchester Victoria have been cancelled.

Mr Hopkins said a "fast-moving investigation" had established the attack was conducted by one man, although detectives are working to establish if he was "was acting alone or as part of a network".

"The attacker, I can confirm, died at the arena.

"We believe the attacker was carrying an improvised explosive device which he detonated, causing this atrocity."

British Transport Police said the explosion happened within the foyer area of the stadium at about 10.30pm.

Just before 1am, the North West Counter Terrorism Unit confirmed to Sky News they are treating this as a possible terrorist incident.

Witnesses reported scenes of panic, confusion and smoke inside the building.

Police had earlier said there were already a "number of confirmed fatalities and others injured" and emergency services at the scene after "reports of an explosion".

A Facebook post, by Paula Robinson from Westhoughton, is being shared online in a bid to reunite lost children with their families.

Dawn Price was at the concert with her daughter and their friends.

She said: "I'm really shaken and can't process what happened as so many people were there with children.

"There was a big bang just as it finished and everyone started to run back into the arena.

"Then seconds later people started to run back in from another exit."

The area around the arena was swamped with police and emergency services. Approach roads have been closed by police.

Witnesses said they heard two loud bangs inside the arena and coming from where the bars are located.

Manchester's Victoria Station, which backs onto the arena, has been evacuated and all trains cancelled.

One fan at the concert described how the explosion hit the venue as fans were leaving the building.

Majid Khan, 22, said: "I and my sister, along with a lot of others were seeing Ariana Grande perform at Manchester Arena, and we were all exiting the venue when around 10.40-10.45pm-ish a huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena.

"It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit Trinity Way and that was blocked so everyone was just running to any exit they could find as quickly as they could."

A British Transport Police statement said: "Officers are at Manchester Arena following reports of an explosion within the foyer area of the stadium at 10.30pm this evening. 

Concert-goers and witnesses described the chaos after "huge bomb-like bangs" went off.

Oliver Jones, 17, was at the concert with his 19-year-old sister.

He said: "I was in the toilet and heard a loud bang just after the concert had finished and people had started to leave.

"The bang echoed around the foyer of the arena and people started to run.

"I saw people running and screaming towards one direction and then many were turning around to run back the other way.

"Security was running out as well as the fans and concert goers.

"Reports of blood and people injured."

He added: "In so much shock and panic. You see this on the news all the time and never expect it to happen to you. I just had to run and make sure me and my sister were safe."

Footage posted online by one concert-goer shows fans filing out of the building while an announcer attempts to maintain calm.

The voice can be heard saying: "Ladies and gentleman, please take your time, there's no need to bunch up, there are no problems here - just take your time and keep exiting the building."

A barman at the nearby Steven Charles Snooker Club, who gave his name as Tyler, said he saw people lying on the ground covered in blood.

 "We've had a few people in with panic attacks and in all kinds of disarray," he told Press Association.

"We've got four girls here - trying to get them sorted to get picked up.

"There was a gentleman on the floor with his leg all bleeding and woman with blood down one side of her face.

"We felt something but didn't know what it was - there was a sound like thunder.

"One girl had a panic attack and another had streaming tears, a woman had a heart attack just outside.

"It's a lot of teenagers - they're all in tears."

Suzy Mitchell, 26, whose flat is opposite the venue, reported a huge bang rocking the neighbourhood.

She told the Press Association: "(I) just heard a huge bang from my bed, came out to the front of my apartments (we're on the top floor so have perfect view) and everyone was running away in big crowds.

"The bang was so big I heard it from my room which is at the back of the apartment blocks.

"Currently lots of emergency services going to and from. But can't see anything substantial as of yet except fleeing people and lots of cars."

A woman who said she worked at the arena as staff but did not want to be named said: "All I know is it sounded like a bomb."

Another concert-goer, a woman with her husband and three young children, said as the concert ended here was a loud bang and everyone leaving the venue then rushed back in.

She said: "I just freaked. Everyone started screaming. We did not see any explosion but it smelt bad, like burning."

Several people on the streets who had been to the gig were visibly upset and in tears and did not want to speak.

A middle-aged woman said her partner, aged 32, and daughter, 19, were outside the arena to pick up her younger daughter who attended the concert.

She said: "There was a bang, a massive explosion". They have not been seen or heard of since and are not answering their phones.

Another man who took his daughter to the gig said: "Ariana said: 'Bye bye Manchester!' The lights came on then there was two big noises and people started running and shouting.

"We got in a lift to go up to the next floor to the exits. When the doors opened I saw blood on the floor and two injured people. Blood all over their faces. A lot of people were in shock."

It is thought about 20,000 people were attending the US singer's concert at the arena, many children and teenagers.

A tweet on her twitter account read: 

The 23-year-old US pop singer, who found fame as the child star of Nickelodeon series Victorious, was touring the world when tragedy struck.

In a statement released through her manager, Scooter Braun, Ariana said she was heartbroken by the event.

"Tonight, our hearts are broken. Words cannot express our sorrow for the victims and families harmed in this senseless attack," the statement read.

"We mourn the lives of children and loved ones taken by this cowardly act. We are thankful for the selfless service tonight of Manchester's first responders who rushed towards danger to help save lives."

The Dangerous Woman Tour started in Phoenix, Arizona, on February 3 and she was joined by British pop stars Little Mix during a spell in the US.

She then made stops in Dublin, Sweden, Norway, Denmark and the Netherlands.

After Manchester she was due to play two nights in London's O2 Arena before visiting Poland, Germany, Switzerland, France, Spain and Italy.

Theresa May said her thoughts were with the victims and families of those affected in the Manchester incident, which is being treated as an "appalling terrorist attack".

The Prime Minister said the authorities were working to establish what happened at the Manchester Arena.

Mrs May said: "We are working to establish the full details of what is being treated by the police as an appalling terrorist attack.

"All our thoughts are with the victims and the families of those who have been affected."

She will chair a meeting of the Government's emergency committee Cobra to respond to the suspected Manchester terror attack.

A Tory source said the Prime Minister's General Election campaign was being suspended.

Liverpool City Region metro mayor Steve Rotheram tweeted: "My 2 daughters caught up in the Manchester explosion at the arena. They are thankfully safe, but I fear for others."

He posted later: "Confirmed deaths at the Manchester arena. Feel sick to think that people have lost their lives at a gig attended by so many young people.

"All my thoughts go out to those parents waiting to hear of the safety of their children. It's a parents worse nightmare. So, so sad."

Mr Rotheram thanked the taxi driver who picked up his two girls from the venue.

Witnesses reported helicopters were circling above the scene after reports of the blast.

Armed police swamped the streets around the arena and police were moving members of the public away from the area.

Just after 1.30am police said: "There will be a controlled explosion in Cathedral Gardens shortly if you hearing anything don't be concerned."

Police later said 'abandoned clothing, not a suspicious item'.

Labour leader Jeremy Corbyn said his thoughts were with all those affected by the "terrible incident" in Manchester following a reported explosion at a packed concert.

His political rival Tim Farron, the Liberal Democrat leader, condemned the "shocking and horrific" attack targeting youngsters at the concert.

In a message on Twitter Mr Corbyn said: "Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services."

Mr Farron said: "This is a shocking and horrific attack targeting children and young people who were simply enjoying a concert.

"My deepest sympathies are with the victims, and with families who have lost loved ones, as well as those desperately waiting for news.

"I would like to pay tribute to the bravery and dedication of the emergency services.

"This is an attack on innocent people and the nation is united both in its grief and its determination to stand up to this deplorable attack."

Labour's Lucy Powell, who is hoping to retain the Manchester Central seat, said she had spoken to Greater Manchester mayor Andy Burnham about the incident.

"This is a major incident targeted at our city," she said. "Our hearts go out to all and our gratitude to emergency services."

The world of music has been sending messages of support after the incident.

A Manchester Arena statement said: "We can confirm there was an incident as people were leaving the Ariana Grande show.

"The incident took place outside the venue in a public space.

"Our thoughts and prayers go out to the victims."

National Rail said: "As Manchester Victoria is located near the arena, the station has been evacuated and all lines closed.

"Trains are currently unable to run to/from Manchester Victoria. Some trains will be cancelled throughout or start/terminate at alternative stations. Disruption is expected to continue until end of the day."

How terror unfolded at the Manchester Arena

Monday, 6pm: Doors open at Manchester Arena in the city centre. US pop singer Ariana Grande is on the bill for a sell-out concert at the 21,000-capacity venue. Fans, many of them teenage girls, excitedly share selfies as they wait for the show to begin.

7.30pm: Show's scheduled start time. After a performance from support act BIA, an American rapper, Grande takes the stage.

10.30pm Show's scheduled finish. Thousands of fans begin to file out of the arena's four exits.

A "huge bomb-like bang" is heard at the arena. Witnesses describe being knocked from their feet by an explosion and seeing dozens of injured, possibly dead, people including children lying on the floor. Others describe panic as concert-goers run for exits.

10.35pm: GMP are called to reports of an explosion at Manchester Arena. Dozens of emergency services vehicles stream into the area.

10.55pm: Police urge people to stay away from the area as responders deal with a "serious incident".

11.46pm: Police say there have been a number of confirmed fatalities.

Tuesday, 1.10am: Nineteen people are confirmed dead and around 50 others injured following the suspected explosion police say is being treated as a terrorist incident.

1.30am: A controlled explosion is carried out on a suspicious item in the Cathedral Gardens area near Manchester Arena. It is later confirmed to be abandoned clothing.

2.15am Prime Minister Theresa May says her thoughts are with the victims and families of those affected in "what is being treated by the police as an appalling terrorist attack".