The primary transit system in the San Francisco Bay Area was forced to shut down all trains on Friday morning because of a computer problem, sending commuters scrambling to find alternatives with little notice.

Bay Area Rapid Transit, which transports more than 170,000 passengers on weekdays throughout much of the region, said service had been suspended because of a “computer networking” problem. Riders were searching for alternatives, from car pools to local bus services and ferries.

The outage came as BART has struggled mightily to attract passengers after the Covid-19 pandemic. Many residents in the tech-driven Bay Area have been able to continue working remotely, and BART has faced one of the steepest declines in ridership of any major U.S. transit system. Passenger traffic on the Bay Area system remains less than half of what it was before the pandemic, according to the American Public Transportation Association.

It was not immediately clear what had caused the networking problem or when service was expected to resume on Friday. Shortly after 7 a.m. local time, traffic was thick at the freeway to the Bay Bridge, which connects Oakland and other East Bay communities to San Francisco.

Chris Filippi, a spokesman for BART, said in a statement that the computer issues were affecting all 50 stations within the system. The last time BART faced a similar shutdown, Mr. Filippi said, the issue took several hours to resolve.

At the El Cerrito Plaza station in the East Bay city of El Cerrito, Calif., a BART official turned away commuters wearing backpacks and clutching big tote bags. She told them to return around 9:30 a.m., when trains between East Bay stations may begin running again.