Electricity was cut for about 40 minutes before it came back on at about 7 p.m., said EDF Energy, which handles some power transmission for London. The outages appeared to be confined to south London and Kent, a county southeast of the city.
London Mayor Ken Livingstone told Sky News that about a half million commuters were affected. He said up to 150,000 people had lost power.
A spokesman for the Underground said 60 percent of the subway system was halted at the height of the evening rush hour, including the majority of services in central London.
Workers were evacuating affected trains and stations. News reports said many passengers were stuck underground, but no accidents or injuries were immediately reported on the train or subway lines.
The Tube can get uncomfortably warm during summer, but temperatures outside were only about 65 degrees.
One woman was stuck outside Victoria Station and said the line for cabs was "about a mile long." Pubs in the area have been packed with stranded commuters.
EDF spokesman Gareth Wynn said the problem originated in two high-voltage lines belonging to the national power grid that help supply the Wimbledon area of southeast London.
He said it was very unlikely the problem had been caused by any kind of sabotage.
A spokesman for the British Transport Police said the outage had affected all of south London's major overland train stations — Victoria, London Bridge, Waterloo — and temporarily halted all the area's main train lines.
"Some stations are in darkness and others have emergency lighting," he said on condition of anonymity before power returned.
The transport police later said power was being restored on some train lines. "Things are now starting to move slowly," the spokesman said.
The police were contacting London Underground to make sure there were no people on the tracks before reactivating the lines.
Kevin Groves, a spokesman for Network Rail, which operates Britain's rail infrastructure, said power had been cut along tracks stretching 20 to 30 miles south of London.
London's Metropolitan Police said 270 sets of traffic lights had gone out in and near south London, but that all had come back on.