LONDON, England (CNN) -- Hundreds of thousands of passengers were trapped on the London Underground and thousands of others took to the streets as a power outage struck the city during evening rush hour Thursday.
Electricity was being restored to parts of London within an hour of the outage, which was blamed on a failure in the National Grid. But the city's transport services were badly affected.
London Mayor Ken Livingstone estimated that 250,000 people were caught in the London Underground when the outage hit at about 6:15 p.m. (1:15 p.m. ET) as commuters were heading home from work.
Another quarter of a million people were affected when parts of the mainline rail system came to a halt, he said.
"There's no indication of any terrorist involvement, but it is an absolutely horrendous position because it has caught the rush hour," Livingstone said.
Thousands of people took to rain-soaked streets as dusk approached and pubs were packed with stranded commuters in scenes reminiscent of the blackout that hit North America earlier this month.
A spokesman for London Underground said 60 percent of the metro system had been affected, including most services in central London.
Network Rail spokesman Kevin Groves said overground rail services had been "badly" affected and the situation was "unprecedented" as far as he knew.
"All power to all our signal systems to the third rail, which supplies power to the trains, all our power to points and lighting have all gone since 6:30 p.m.," Groves told the UK Press Association.
He said a stretch of rail 20 to 30 miles south of London had been affected.
"We have back-up generators but they can't be used in a power failure of this magnitude. You practically need a power station."
A spokesman for British Transport Police said the power cut had affected all major railway stations in south London.
Commuters wait outside a closed Underground station.
"There is no power in a lot of stations and no trains running in the whole of south London," he told PA.
"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.
London Underground spokeswoman said staff were taking passengers off trains stuck in tunnels.
"Stations and trains are being evacuated. ... Staff will go down to the train and take (passengers) down the tunnel bit by bit and make sure they get onto the platform and leave the station safely," she told PA.
She said some stations had been closed and 250 sets of traffic lights had also been hit by the blackout.
National Grid failure
A spokesman for electricity network operator EDF Energy said: "We have lost supplies to large parts of south London ... as a result of a National Grid failure supply in the south London area."
French-owned EDF Energy said electricity was out for about 40 minutes before it returned at about 7 p.m.
EDF spokesman Gareth Wynn said the problem originated in two high-voltage lines belonging to the National Grid that help supply the Wimbledon area of southeast London, The Associated Press reported.
He said it was very unlikely the problem had been caused by any kind of sabotage, and a Scotland Yard spokesman said there was "no suggestion there's anything suspicious."
British energy regulator Ofgem said the fault lay with National Grid Transco, which runs Britain's power lines.
"There was a fault on a transformer system. There was loss of power for 34 minutes, but London Electricity has re-energized the system and it's back on," Reuters quoted an Ofgem spokesman as saying.
National Grid's U.S. business Niagara Mohawk was among those hardest hit by the blackouts that struck the east coast of North America earlier this month.
National Grid said it did not immediately know the cause of the outage, which it said was a loss of 700 megawatts -- "not even vaguely on the scale of what happened in the U.S."
Thursday's outages appeared to be confined to south London and Kent, a county southeast of the city.