Some of the most popular sites on the internet went down for several hours yesterday following a power cut in San Fransisco.

Craigslist, the classified advertising site, Typepad, the blogging platform, and Second Life, the virtual world, were among those affected when a data centre in central San Fransisco was left temporarily without power.

The incident happened on the same day the data centre, 365 Main, distributed a release touting "two complete backup systems for electrical power to protect against a power loss."

Most of the sites which went down, including Technorati, the blog-tracker, were back up after a couple of hours, but not before millions of users tried and failed to access them during the early afternoon, local time.

"At approximately 1.45pm, the 365 Main San Fransisco data centre experienced a massive power outage," a note on the Craigslist site read. "Power to our server, which is located in this facility was interrupted several times." At 1.12am, an update said: "We're cautiously optimistic we get to go to bed soon."

A similar message on TypePad read: "We are truly sorry for the frustration and inconvenience that you've experienced. We also appreciate the commiseration from the teams at many of the other sites that were affected."

A spokesman for 365 Main said that the company switched to backup power, but confirmed that some customers were temporarily affected by the outage.

The Pacific Gas & Electric Company said on its website that 40,000 San Fransisco businesses and residents were affected by power fluctuations caused by an open circuit breaker. It said that it was still investigating what caused the fault, which took two hours to rectify.

"It was right in the middle of the US workday, so it was very bad for us," Derek Gordon, an executive at Technorati told the LA Times.

"It exposed a larger vulnerability," Mr Gordon added. "If this could happen to such a collection of major websites, what would happen if this was part of a major catastrophe? This was sort of a wake-up call."

Data centres are typically large warehouses which are home to enormous racks of servers on which businesses store their information.

Many internet businesses, including Google, have indicated they intend to provide more and more services based on storing customer information, underlining the importance that such centres should be able to cope with occasional outages.

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