(CNN) -- Public transit in San Francisco came to a screeching halt Monday morning as Bay Area Rapid Transit unions went on strike.

The strike began at midnight (3 a.m. ET) Monday after union leaders walked out of last-minute pay negotiations with BART, hours before their four-year contract was set to expire.

About 400,000 commuters use BART every day in the San Francisco Bay area, BART spokesman Rick Rice said.

"The public doesn't deserve to be punished," Rice said. "We are sorry they have decided to strike despite the fact we are willing to negotiate. The district is prepared to return to talks and get this finished."

Josie Mooney, spokeswoman for SEIU Local 1021, also lamented the impact on the community.

"A strike is always the last resort, and we have done everything in our power to avoid it. Unfortunately, BART seems intent on forcing a strike," Mooney said in a statement. "We are disappointed that BART's failure to bargain honestly and fairly means that hundreds of thousands of Bay Area commuters have to suffer."

At odds

The dispute centers around pay and benefits.

"They've asked for (a) 21 percent pay increase," Rice said. "We've gone from offering 4% over four years to 8percent over four years."

Rice also said BART also offered to reduce the amount of employee contributions originally requested for pension and medical benefits.

But local SEIU union President Roxanne Sanchez said workers are fed up.

"Years of layoffs have affected public safety and services. Crippling cuts have not just made our jobs more difficult, but put undue strain on our livelihoods, our families and our communities," she said in a statement.

"Believing in good jobs that pay fair wages, offer health care and a secure retirement --- these are modest ideals. These are UNION ideals."

CNN's John Branch contributed to this report.

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