A power outage hit downtown San Francisco Tuesday afternoon local time, leaving the area without power and knocking popular Web sites such as Craigslist, GameSpot, Yelp, Technorati, TypePad and Netflix offline for a few hours.

The power failure apparently hit 365 Main, a 227,000-square-foot datacentre in downtown San Francisco, particularly hard. The data co-location centre's client list includes Craigslist, and CNET Networks' GameSpot.

It wasn't immediately clear if the other affected Web sites were customers of 365 Main or of other Web hosting companies, or whether the sites were blacked out for all visitors.

At 4 pm (local time) in an e-mailed statement, 365 Main's vice president of marketing, Miles Kelly, had this to say: "At 1:45 pm today, there was a major power event in San Francisco that impacted business operations for many San Francisco based companies, including 365 Main's San Francisco datacentre. PG&E has not yet determined the cause of the failure. Some customers within the 365 Main facility were temporarily affected by the utility failure. The building is currently 100 percent operational and running on backup power [generators] until the company can confirm that utility power is stable."

Most sites seemed to be working again by 4:45 pm (local time). What this means for 365 Main's service agreements with its customers, which promises 24-hour-a-day, 7-days-a-week, 365-days-a-year power, is still unclear. We're waiting to hear more from 365 Main. In an ironic note, a press release from 365 Main dated Tuesday noted the company had provided Red Envelope "two years of continuous uptime."

Asked why the datacentre company, which bills itself as "The world's finest data centres", failed to meet its standard of ensuring service to customers even in the event of a power failure, Kelly said "I don't know".

According to the San Francisco Chronicle, Pacific Gas & Electric said that more than 30,000 of its customers lost power after an explosion under a manhole cover on Mission Street.

Contrary to prior reports, there was no mob of angry customers outside the 365 Main building, and no drunk employee had gone on a rampage, according to Kelly. The "mob" was actually a line of customers who were forced to enter through the front door and have badges checked manually to get into the building because the parking garage gate was affected by the power outage, according to Chris Hutchens, a network engineer at SF Data, which is a customer of 365 Main.