The union which represents ambulance workers in Yorkshire says plans to change shifts could affect patient safety.
Workers across the county walked out at midnight for a 24 hour strike.
Yorkshire Ambulance Service NHS Trust refutes the claims made by Unite.
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 chief executive of the Yorkshire Ambulance Service NHS Trust said the industrial action would affect services, involving around 8% of its staff.
He refuted claims by the workers' union, Unite, that changes in shift patterns would affect patient safety.
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.
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.
– Yorkshire Ambulance Service NHS Trust chief executive David Whiting
Mr Whiting added the service will take steps to 'maintain operational cover' during the strikes, and will 'continue to make every effort to get to patients as quickly as possible whilst maintaining high standards of patient care.'