San Francisco's public transit service, BART, stopped running early Saturday, halting all train traffic for several hours before "limited service" was restored.

Bay Area Rapid Transit said Saturday that crews working overnight on BART's uninterruptible power supply ran into problems that impacted the power supply system and train control routing system. Without these systems, a BART statement said, it is unable to safely dispatch its trains for service.

"We are now open with limited service," BART spokesperson Alicia Trost told CNBC via email about 9 a.m. local time.

The transit agency announced earlier it had multiple crews working to get the problem resolved, but for now, it arranged for bus agencies to provide free trips for service between stations.

BART's ridership on weekdays averages more than 430,000 and it connects downtown San Francisco with the East Bay region cities of Oakland, Berkeley, Fremont and other communities. The transit system also provides trips to SFO, the San Francisco airport, for about 4 million people annually.

Some regular BART riders trying to get to work Saturday vented their frustration about the outage, taking to Twitter and suggesting they would need to resort to ride share services such as Uber.

https://twitter.com/gaperez10/status/1104414502890627072

There also were commuters who refused to use services such as Uber due to cost and decided to return home from planned trips after learning about the BART shutdown.

https://twitter.com/mikaelawhy/status/1104408789996859392

This story is developing. Please check back for updates.