LOS ANGELES (AP) — University of California medical center workers began a two-day strike Tuesday that could involve thousands of employees and prompted postponement of some surgeries.

Striking workers whistled on the picket line at UC San Francisco. (Andrew Stelzer/KQED)

The union representing some 13,000 hospital pharmacists, nursing assistants, operating room scrubs and other health care workers began the walkout at 4 a.m. Tuesday at medical facilities in cities including San Diego, Orange, Los Angeles, Santa Monica, San Francisco and Sacramento, home to UC Davis Medical Center.

Green-shirted picketers were in place at the Los Angeles facility.

The union is battling over staffing and pension issues.

"We care about our patients and we feel that we're chronically understaffed and we need additional help," Ruben Gomez, a radiation therapist in Los Angeles, told KCBS-TV.

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More than 2,000 workers were expected to walk out at UC facilities in Los Angeles and Santa Monica, but the strikers set up a task force to ensure that critically ill patients receive care, Gomez said.

University officials prepared for the strike by postponing nonessential services, including some surgeries and some chemotherapy sessions for children, and by bringing in temporary workers.

"There may be delays," said Dr. Tom Rosenthal, chief medical officer for UCLA Hospitals. "Things are likely to be way slower than on a normal day. But things will be safe."

The workers, represented by the American Federation of State, County and Municipal Employees, contend that UC officials have allowed executive pensions to balloon, diverting money that would otherwise go to alleviate dangerously low staffing levels.

System officials said the main issue is the union's refusal to accept a new pension plan, similar to those of other state workers, that requires more employee contributions and reduces long-term benefits for new hires.

They had sought a judge's injunction to stop the strike for the sake of public safety, but on Monday Sacramento County Superior Court Judge David Brown granted it only partially, ordering 453 workers to remain on the job.

"If all respiratory therapists in the UC Burn Centers and all pharmacists working in UCSF's California Poison Control System were permitted to strike," the judge wrote in his ruling, "there is reasonable cause to believe that the strike would prevent delivery of an essential public service."

But Brown also said there was no reason that others could not strike as long as staffing remained at weekend levels for critical services.

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Workers Start 2-Day Strike at UC Hospitals