Thousands of service workers at the University of California went on strike Monday to call for higher wages.

Custodians, security guards and other university employees at the system’s 10 campuses are striking for three days, according to The Associated Press.

The union representing the workers, the American Federation of State, County and Municipal Employees (AFSCME) Local 3299, called for the strike after a contract dispute with the university system. The chapter represents 25,000 workers.

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Nearly 30,000 medical workers from the university’s five medical centers are expected to join the last two days of the strike in solidarity with the university employees. That will force some hospitals to stop non-emergency procedures.

Photos and video of the strike from the AFSCME and local news outlets show workers wearing green shirts and carrying signs that call for “Equality, fairness, respect.”

A spokesman for the union told the AP that pay inequality for the workers has been driven by the university “actively seeking to hire contract workers.”

The union is demanding a nearly 20 percent pay raise over three years, a university official told the AP.