Sporadic power failures hit the city Tuesday, affecting a broad swath of downtown, including the arena where the San Francisco Giants play.
About 51,000 homes and businesses were without electricity at the height of the outage, which started at about 2 p.m., said Pacific Gas & Electric spokeswoman Darlene Chiu.
Many were successfully rerouted to backup power circuits, but about 30,000 customers remained in the dark an hour later as PG&E crews worked to identify the outage's cause, Chiu said.
"Our priority now is to restore service back to our customers as fast as we can," she said.
AT&T Park, home of the Giants, was also affected hours before a scheduled night game.
Six Apart Inc. said its sites began failing shortly before 2 p.m., and the company sent an e-mail to customers blaming the city's "power issues." The San Francisco-based blog hosting service reported that power had resumed shortly afterward, but employees were checking data and computers.
Several other Internet sites with offices in San Francisco had problems Tuesday afternoon, including Technorati, Yelp, Red Envelope and CNet. It was unclear whether the problems were related to the outage.
San Francisco-based classified site Craigslist.org was down Tuesday afternoon, but founder Craig Newmark said he didn't know why.
Bay Area Rapid Transit trains were operating normally, though one downtown station was running on backup batteries, a BART spokesman said. The San Francisco Police Department said it had not received additional calls related to the problem.
Gregg Fishman, a spokesman for California's Independent System Operator, which manages the state's power grid, said there had been no reports of widespread failures outside San Francisco.
"There's nothing that we know of on our system that would affect San Francisco," Fishman said.