LONDON — Hospital doctors in England staged their first strike in four decades Tuesday, disrupting treatment for thousands of patients in the National Health Service and escalating political tensions over a publicly funded health care system so revered that it was once likened to a national religion.

Operations were postponed and appointments canceled in a bitter dispute over pay and working hours between employers and junior doctors, a term that covers medical professionals with as much as a decade of experience.

With the junior doctors offering only emergency care, about 3,500 operations had been affected by Tuesday afternoon, including routine procedures for knee and hip replacements — prompting a warning from Prime Minister David Cameron that the labor action would create “real difficulties for patients, and potentially worse.’’

Yet the dispute over the health system carries risks for the government. The National Health Service, which is funded by taxes and payroll deductions but has faced years of financial strain, delivers most treatment without charge. Despite regular funding crises, there has been no similar strike since 1975.

Cameron’s Conservative Party has always found it hard to make changes to the health service, which was created by the Labour Party in the 1940s and is now creaking under the strain of an aging population and tightened budgets.

In his memoirs, Nigel Lawson, a chancellor of the Exchequer under Prime Minister Margaret Thatcher, wrote that health practitioners regarded themselves as “a priesthood,’’ making the sector “extraordinarily difficult to reform.’’

The National Health Service, he wrote, “is the closest thing the English have to a religion.’’

It is also a significant presence in national life, employing 1.6 million people.

Weekend shifts are at the heart of the current dispute. A proposed new contract would increase basic pay but would reduce the number of hours for which junior doctors receive added compensation for work, particularly on Saturdays.

The government argues that this would improve treatment by creating a genuine seven-day service, and the health secretary, Jeremy Hunt, on Tuesday highlighted the elevated mortality rates recorded for some medical conditions on weekends, when hospitals have fewer staff.

Speaking to the BBC, Hunt compared the situation of a hospital doctor to that of an airline pilot’s being told, “‘I’m sorry, but as it’s Sunday you don’t have a copilot, but off you go to New York.’’’

The doctors counter that their stand against excessive working, and the strain it puts on them, makes them the guardians of safety in hospitals.

Officially, junior doctors are required to work a 48-hour week, but that is calculated over a 26-week period, and they can end up working long stretches, particularly over weekends.