UNION CITY (CBS SF) — A massive power outage early Saturday evening disrupted electrical service to at least 50,000 customers in Alameda County, mostly homes in Union City and Newark, for several hours.

At the height of the outage, Pacific Gas & Electric officials said there were 33,000 customers without power in Union City and 17,000 in Newark. They lost electrical service at around 6:54 p.m.

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The utility said crews were able to restore power in Union City at 9:10 p.m. and to the Newark homes by 9:30 p.m.

Officials had warned motorists to be aware that stop signs may not be working.

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“Traffic signals not functioning,” the Union City police said in a release. “Railroad arms stuck in down position. Treat non-functioning signals as a stop sign.”

The power outage also forced BART officials to shut down the South Hayward Station and put a bus bridge in place.

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Crews were still trying to determine the cause of the outage.