LONDON (AP) — Thousands of junior doctors walked off the job Tuesday in England in a bitter dispute over pay and working conditions — the first such strike in 40 years.

About 50,000 junior doctors —those who are training and have between one and 10 years of experience — were on strike for 24 hours protesting government plans to change pay and work schedules.

The striking doctors argue patients will be put at risk by the government's policies, while the government says the National Health Service needs more flexibility to deliver services on weekends.

The strike has forced the cancellation of about 4,000 operations and outpatient procedures.

Prime Minister David Cameron pleaded with doctors on Monday to call off the action, and Health Secretary Jeremy Hunt urged union members to return to the negotiating table for new talks with the government. The public health service is considered a treasured institution here and enjoys widespread support despite its many problems.

Britain's government has insisted that the health service has been ring-fenced from the cuts hitting other government agencies as part of austerity plans meant to control the budget hit hard by the 2008 financial crisis. But with medical costs rising, the government insists changes are needed, particularly in staffing on weekends.

Waving banners saying "The NHS needs saving and they're not listening but we've got something to say," demonstrators formed picket lines outside hospitals beginning in the early morning.

Nadia Masood, 34, who organized the picket at Great Ormond Street Hospital, accused the government of not negotiating in good faith with public sector workers. Masood said support for the strike among the doctors was overwhelming.

"This isn't the first time we have been mistreated by the government though, that just tells you how bad these changes are," she said.

Singers serenaded the strikers, and cab drivers honked horns in support.