San Francisco firefighters responded to the scene right away and helped clear the smoke. PG&E crews arrived on scene around 11 p.m.
At the time of the explosion, there was the Green Build reception going on in the rotunda and they had to be evacuated.
"We had an electrical explosion in the basement in the electrical room and PG&E just arrived. There is minimal smoke throughout the building. It's a high voltage area, so we can't get anybody inside there until we get the power cut off," said San Francisco Fire Asst. Chief Art Kenney.
"It was a pretty calm scene, the alarms went off, and the lights started flickering, the sheriffs came in and just asked people to exit the building. I never did smell smoke, not until we actually go out of the building," said caterer Michael Kernan.
There were no injuries reported. Firefighters say it will take PG&E about two hours to shut off the power because there are several lines going into City Hall and they don't want to turn off the wrong line and perhaps cut off power to a portion of the city.
Kernan said City Hall is expected to be open on Wednesday for regular business hours.
Wednesday morning update: San Francisco officials announced that the third and fourth floors of city hall will not re-open until power is restored this afternoon. The building is running on generator power.