Thousands of junior doctors have walked off the job in England in a dispute over pay and working conditions — the first such strike in 40 years.

Some 50,000 junior doctors — who represent a third of the medical workforce — are on strike for 24 hours protesting government plans to change pay and work schedules. The doctors argue patients will be put at risk, while the government says the National Health Service needs more flexibility to deliver services on weekends.

The strike Tuesday has forced cancellation of some 4,000 operations and outpatient procedures.

Prime Minister David Cameron had pleaded with doctors to call off the action. But Dr. Kitty Mohan, from the British Medical Association's junior doctors committee, told the BBC that doctors "really need proper contractual safeguards."