The first strike by medical doctors in England in four decades disrupted care for tens of thousands of patients on Tuesday, heightening tensions over stewardship of a widely revered health system that has come under growing strain.

Thousands of junior doctors, a term that covers medical professionals with as much as a decade of experience, were providing only emergency cover because of a dispute over pay and working conditions, notably weekend shifts.

The strike led to the cancellation of around 4,000 nonurgent operations, such as routine procedures for knee and hip replacements, and Prime Minister David Cameron warned that the strike would create “real difficulties for patients and potentially worse.”

The popularity of the program means that the dispute carries risks for the government. The National Health Service,, which is financed by taxes and payroll deductions, delivers most care for free. There has been no similar industrial action since 1975.

Mr. Cameron’s Conservative Party has always found it hard to put in effect 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 Margaret Thatcher, wrote that health practitioners regarded themselves as “a priesthood,” making the sector “extraordinarily difficult to reform.”

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

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

The government argues that this would improve treatment on weekends by creating a genuine seven-day service.

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

To an extent, the dispute has crystallized a broader set of worries and frustrations felt by many doctors working in a system in which demand for health care sometimes seems infinite, but for which resources are definitely not.

When junior doctors were asked to authorize a strike last year, 98 percent voted in favor, and a threatened strike in December was postponed at the last minute.

Some opinion surveys have suggested that public support lies with the medical professionals, at least in the initial phase of the walkout. The doctors are planning two more protests in the coming weeks: a 48-hour strike that would also affect nonurgent care, and another day’s walkout that would mean them withdrawing all treatment.

Plans for the new contract have been under discussion for several years, but the government has indicated that it would impose the new terms in the absence of an agreement.

The government said that doctors would not be worse off under the new contract. That assertion has been disputed by the British Medical Association, which represents more than 37,000 of the country’s 55,000 junior doctors and which describes the conditions created by the contract as “unsafe and unfair.”

The financial implications of any change would be significant because of the huge number of people employed by the health service. The N.H.S. says it employs 1.6 million people, putting it in the top five of the world’s largest workforces, alongside the United States Department of Defense, McDonald’s, Walmart and the Chinese People’s Liberation Army.

Johann Malawana, who leads the medical association’s junior doctors committee, said that the members “feel they have been left with no option but to take this action.”

“Our door is open to talks, but the government must address our concerns around safe working patterns and ensure the contract recognizes the long, intense and unsocial hours which junior doctors do,” he said in a statement.

The Department of Health said it was working with the health service to minimize the impact to patients during the strike action.

N.H.S. England, which leads the country’s health care system, said it regretted that thousands of planned procedures would have to be rescheduled, and that people would have to wait longer for treatment as a result.

“We have tried and tested plans to deal with a range of disruptions including industrial action,” Anne Rainsberry, a director of N.H.S. England, said in a statement. “As ever, the safety and care of patients is our top priority, and the N.H.S. has robust plans in place to ensure those who need emergency treatment will continue to receive it.”