From CNN.com

LONDON, England -- Passengers were trapped on the London Underground as a power outage struck the city during evening rush hour Thursday.

"We have lost supplies to large parts of south London in the last few minutes as a result of a National Grid failure supply in the south London area," a spokesman for electricity network operator EDF Energy told Britain's Press Association.

"It's difficult to predict how long this is going to take. National Grid has got to get the circuit back."

A spokesman for London Underground said 60 percent of the subway system had been halted by the outage, including the majority of services in central London. The failure happened around 6:15 p.m.

By 7:30 p.m., Britain's energy regular Ofgem said power was returning to parts of London.

"There was a fault on a transformer system. There was loss of power for 34 minutes, but London Electricity has re-energised the system and it's back on," Reuters quoted an Ofgen spokesman as saying.

Thousands of people took to the streets as dusk approached. Street lights in some parts of London were not working in scenes reminiscent of the blackout that hit North America earlier this month.

London Underground said the power failure was having a "serious" impact on the whole of the city's metro network. Rail services from major stations including Victoria, London Bridge and Waterloo were also affected.

"There has been a National Grid failure which has affected the underground. At the moment we are assessing the impact of that, but it is having a serious impact on the whole system at the moment," a spokesman for the London Underground said.

He added: "It's also having an affect on some of our buildings. Certainly quite a lot of the network appears to be affected."

"There will be travelers underground. I don't know the extent of that yet. It's at the end of rush hour, the peak period, so it's a busy time."

An Underground spokesman told Reuters: "Trains in stations are being evacuated. We're trying to get alternative power sources started."

A spokesman for British Transport Police said the power cut had affected all major railway stations in south London.

"There is no power in a lot of stations and no trains running in the whole of south London," he said. "All major stations -- Victoria, London Bridge, Waterloo -- are affected and all main train lines have stopped. Some stations are in darkness and others have emergency lighting."

Euston Station was evacuated because of overcrowding, apparently because passengers could not get on the Underground.