Thousands of employees walked off the job at five University of California (UC) medical centers on Tuesday, kicking off a planned two-day strike that aims to highlight purported staffing shortages at UC hospitals.

The American Federation of State, County and Municipal Employees, which represents almost 13,000 respiratory therapists, radiology technologists and vocational nurses, say that the strike was motivated by concerns over patient care. According to the striking workers, low staffing levels prevent them from devoting adequate time to all of their patients.

But hospital officials contend that the workers are really upset over proposed changes to their pension system, which other UC employees have already agreed to. A contract suggested by the university would require employees to pay more in contributions to their pension funds and would also change eligibility rules for retiree healthcare benefits, Thomson Reuters reports.

Although some workers chose to come to work despite the strike, the walkout has led to the postponement of hundreds of non-emergency surgeries and other procedures, the Los Angeles Times reports. Some of the medical facilities were diverting ambulances away from their emergency rooms, but others continued to operate as usual.

The strike, which affects UC medical centers in Los Angeles, San Diego, San Francisco, Sacramento and Irvine, is scheduled to end on Thursday morning.

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