The walkout began at midnight after union leaders left last-minute negotiations on pay with BART just hours before their four-year contract was to expire, CNN reported.
"The public doesn't deserve to be punished," BART spokesman Rick 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 Service Employees International Union Local 1021, said the strike was a last resort and also bemoaned its impact.
"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."
Rice said the SEIU is seeking a 21 percent pay increase over the life of the contract and BART has raised its offer of 4 percent to 8 percent. Rice said BART also offered to reduce the amount of employee contributions for pension and medical benefits from the authority's original request.
SEIU Local 1021 President Roxanne Sanchez said workers have had it.
"Years of layoffs have affected public safety and services," Sanchez said in a statement. "Crippling cuts have not just made our jobs more difficult but put undue strain on our livelihoods, our families and our communities."