A major power outage struck San Francisco on Friday morning, affecting at least 22,000 PG&E customers at its peak, after construction crews struck an underground power line, the utility said.

The outage, first reported just before 8 a.m., left customers in Potrero, South of Market and China Basin in the dark, according to PG&E.

As a result, traffic signals were not working in Mission Bay and other areas, police said. And MUNI trains had to be operated manually, the SFMTA tweeted.

It appears a third-party construction company that's not affiliated with PG&E struck an underground power line while doing construction near Illinois and 22nd Street, the utility said. PG&E first reported that the crews hit the line at Pier 17.

At first, PG&E said the utility might be able to restore the power by 10:30 a.m., but then a spokeswoman said she didn't know when all would be fixed.

As of 11:30 a.m., all but two electric customers had their power restored. Most customers saw their power back on by 10:30 a.m.