SAN FRANCISCO—Parts of San Francisco City Hall remained without power on Wednesday morning after a small electrical fire the previous night.

The fire broke out around 9:30 p.m. in an electrical vault and sent smoke into the building's main rotunda. Hundreds of people at a reception there were evacuated.

Christine Falvey, a spokeswoman for the mayor, said automatic sprinklers in the vault were activated. The fire was put out quickly, and the building suffered only minor damage.

City Hall was open for business as usual on Wednesday despite the power outages.

It's not clear what caused the blaze.