About 15,000 medical workers on Tuesday started a three-day strike at five University of California medical centers amid a dispute over pay raises and job security, forcing the cancellation and rescheduling of thousands of surgeries and outpatient appointments, officials said.

The workers that included radiology technicians, respiratory therapists and pharmacy workers picketed the medical centers in Los Angeles, San Francisco, San Diego, Irvine, and Davis. They rang bells, wore green-shirts and carried signed that said “End Outsourcing.”

Another 24,000 other California union workers, ranging from truck drivers to gardeners and cooks, were striking in sympathy, said John de los Angeles, spokesman for the American Federation of State, County and Municipal Employees Local 3299.

Emergency rooms remained open at the hospitals but officials said the strike would still affect thousands of patients.

The UC San Francisco hospital rescheduled more than 4,000 appointments at its medical center and two associated clinics and rescheduled 241 surgeries, including “pretty high-risk” gynecological and colorectal operations, said chief operating officer Sheila Antrum.

Ten patients, including children, were sent to other hospitals, she said.

The hospital had 500 temporary employee replacements on hand, including respiratory therapists, social workers, pharmacists and housekeepers, she said.

“My focus here is that we get through everything with the usual great care we have,” she said.

The union said some of its workers would keep working despite the strike to ensure patient safety, and it also has a “patient protection task force,” a group of emergency services staffers who are prepared to cross the picket lines if they are needed to work because of a medical crisis, de los Angeles said.

Patient care workers have been without a contract since December. Talks and mediation efforts have stalled, and the university plans to impose new employment terms next month, de los Angeles said.

The union wants the university to stop outsourcing low-wage work that it claims is fueling widening income, racial and gender gaps for workers at UC’s hospitals, clinics campuses and research facilities.

The same issue prompted a three-day walkout by 53,000 UC workers last May, including custodians and cafeteria workers. Nurses and other medical workers walked out then in sympathy.

“They’re destroying what were once career pathways to the middle-class for our state’s diverse population and are damaging the quality of service that we provide to students, patients, and everyday Californians,” Monica De Leon, vice president of AFSCME Local 3299’s Patient Care Technical Unit, said in a statement.

A University of California statement said union leaders spread false information about outside service contracts and that the number of unionized patient care workers increased nearly 19 percent over the past five years while outsourcing contracts had stayed relatively flat.

The employment deal to be implemented next month would grant 3-percent-a-year raises for the next 4 years for patient care and service staff, as well as offering a health plan at the same rates as other UC employees with similar salaries.

The union, however, is demanding an “unrealistic and unreasonable” 8 percent annual wage increase that is nearly triple what other UC employees have received, UC Office of the President spokeswoman Claire Doan said in a statement.

The statement accused union leaders of “throwing a tantrum … putting their agenda above the needs of patients, students and the public.”

But de los Angeles, the union spokesman, said the new employment terms do nothing to deal with the threat of replacing union jobs with cheaper outside contractors who are ill-paid and lack benefits.

“What good is a raise if our job is allowed to be outsourced tomorrow?” he said.