Ambulance workers are staging strike after claiming a change on shift patterns will put patients' safety at risk.
Members of the Unite union in Yorkshire walked out for 24 hours at midnight, and will do so again for four hours from 3pm on Monday.
They claim some paramedics could work for 10 hours without a meal break under new longer shift patterns.
A health chief has attacked the strikes as "reckless", but t he union said the changes will impact on patient safety and is calling for a 30-minute meal break every six hours.
The Yorkshire Ambulance Service NHS Trust said the industrial action would affect services, involving around 8% of its staff.
Chief executive David Whiting accused Unite of "neglecting" patient care, adding: " I would like to reiterate that I remain deeply concerned over this type of action - which I know is of concern to all of our A&E staff, who are very committed to patient care - and will place many of them in a very difficult situation.
"We continue to refute the misleading and factually incorrect claims being made by Unite around patient safety.
"Our focus is on safeguarding patient care, and we are committed to minimising the level of disruption to our services. Industrial action in this form is certainly not in the best interests of patients, and it is deeply concerning for a trade union representing ambulance service workers to strike without making any concessions to patient safety.
"Throughout the 24-hour period of the strike we will be taking steps to maintain operational cover to sustain effective and safe services for patients calling upon us for emergency medical assistance. We will continue to make every effort to get to patients as quickly as possible whilst maintaining high standards of patient care.
"Our contingency plans are focused on providing a safe, responsive and high-quality emergency service to patients and this will always remain our top priority."
Unite officer Terry Cunliffe said: "Our members, who are doing their best for the Yorkshire public in very difficult circumstances, have been under sustained attack by the trust's hardline management for more than a year.
"The latest erosion in their employment conditions is the demand to work elongated shifts, which could mean them working more than 10 hours on the trot before managers deign to give them a meal break. This could affect their ability to do their jobs - helping people in distress.
"I think the people of Yorkshire will find that this is completely unacceptable."
The trust, which is to stop recognising Unite for collective bargaining, receives an average of 2,100 emergency calls a day.