Around 450 ambulance service workers are on strike across Yorkshire in a dispute over cuts in patient care, a trade union said.
Members of Unite who work for Yorkshire Ambulance Service NHS Trust picketed 17 stations across the region.
Unite regional officer Terry Cunliffe said the dispute came after the ambulance service "de-recognised" the union amid a row over how changes to the service would affect patient safety.
He said the dispute surrounded a workforce plan based on £46 million of cuts and plans to introduce a care assistant role - the training for which would be six weeks - to replace ambulance technicians.
"We say that's not sufficient to turn up to emergencies and provide life-saving patient care," Mr Cunliffe said on a picket line at ambulance headquarters in Wakefield, West Yorkshire.
"It's not a political decision. I'm not sure what alternative we have. Our members are left with no voice in the National Health Service. That's unacceptable to us and we won't be silenced." Mr Cunliffe warned Yorkshire Ambulance Service NHS Trust: "You must listen to us on patient safety issues."
The majority of ambulance staff, including members of other unions, were continuing to work. Yorkshire Ambulance Service NHS Trust said it expects the action to put pressure on services but provisions have been made.
The walkout, which is believed to include paramedics and support staff, is scheduled to last 24 hours. The Yorkshire Ambulance Service, which employs 4,300 people, said 40 employees had not turned up to work after declaring they were on strike.
However, a trust spokesman said that because the service is a 24-hour, seven-day operation, staff who were not scheduled to work may have been included in the union's estimate.
Paul Mudd, locality director at Yorkshire Ambulance Service NHS Trust, said: "It hasn't affected our service to the public. We're giving the same service we would any other day."