An underground explosion in downtown San Francisco sent black smoke pouring out of a manhole and cut power to 8,800 people. There were no immediate reports of injuries.

Pacific Gas and Electric says the blast occurred around 11:30 a.m. Friday and that crews found an electrical cable burning in a vault. Police cordoned off a two-block area, and firefighters sprayed foam into the manhole.

A power spokesman says the company is waiting for crews to finish extinguishing the fire so it can investigate and get power back on.

Authorities say the problem apparently originated in a transformer but they haven't been able to examine it.

There were no forced evacuations, but authorities advised people to stay inside. Residents were complaining of the horrible smell and some were leaving despite advice to stay put.