Work has been going on through the night to restore power after massive blackouts hit major cities in the eastern United States and Canada.
Many took to the streets after the power went off
The power failures caused chaos as they spread from New York to Detroit, and Toronto to Ottawa.
Traffic lights failed, underground railways were evacuated and people were trapped in lifts in offices and apartments.
Canadian officials said a fire at a power plant near the upstate New York town of Niagara caused the outage, but US officials disputed that theory, although they said terrorism was not to blame.
Power is slowly returning to the affected areas - thought to encompass more than 50 million people - but full restoration will take much longer, officials say.
Federal Energy Regulatory Commission spokesman Bryan Lee said the outage destabilised the power grid, cascading up and taking "a loop around the Great Lakes into Canada".
But US officials were looking at a power transmission problem from Canada as the most likely cause of the biggest outage in US history, said a spokeswoman for New York Governor George Pataki.
In other developments:
- There have been reports of sporadic looting in at least four areas of the Canadian capital, Ottawa.
- President George W Bush said the outages were not a terrorist act and added: "We're slowly but surely coping with this massive national problem."
Nine nuclear reactors in four US states were taken offline, the nuclear regulator said.
The US Federal Aviation Administration halted flights into six airports - three in the New York area, one in Cleveland, and two in Canada - for several hours.
- In New York City, subways, lifts and airports, including John F Kennedy and LaGuardia airports, lost electricity or resorted to limited backup power.
- The blackout closed the Detroit-Windsor Tunnel, which 27,000 vehicles use daily.
The BBC's John Terrett in New York says the city is waking up to an unusual sight - thousands of people lying on the sidewalks, propped up against shop fronts, lounging on the edge of fountains, huddled round small clusters of night lights chatting.
He says the authorities promised to restore power for rush hour but while electricity is returning slowly to the suburbs, the island of Manhattan remains in the dark.
RECENT US BLACKOUTS
1996: 4m people hit by electricity outage across nine states
1977: lightning strike leaves New York without power for 25 hours
1965: power loss in north-east US and southern Canada hits 30m people
As dusk faded into night, New York's famed skyline was shrouded in darkness and residents turned to candles and torches for light.
Many workers left their offices early, walking across bridges out of Manhattan, choking pavements and roads.
Civilians manned intersections directing gridlocked traffic after traffic signals failed, while on the pavements dozens of people queued up to use payphones after mobile phone networks broke down.
New York Mayor Michael Bloomberg said power was starting to come back from the north and from the west but would take hours not minutes.
He told New Yorkers to treat Friday "as a snow day. It wouldn't be the worst thing to take a day off," he said.
In an effort to calm frayed nerves, he cautioned New Yorkers to stay cool and drink fluids.
New York emergency services had no reports of anyone injured during evacuations from tall buildings or underground subways.
Power to many hospitals and prisons was also reported to have failed but key institutions were working on back-up generators.
The BBC's Lee Carter, in Toronto, says the lights went back on in the centre of the city just after 2300 local time (0300 GMT).
The city's landmark, CN Tower, was once again illuminated, but power is returning very slowly to the city, with most areas still reportedly in darkness.
As in New York, the blackout hit Toronto, Canada's commercial and financial centre, right at the beginning of rush hour.
Trams and underground trains in the city came to an abrupt halt.
The situation was made more uncomfortable by the 90F (32C) heat in New York and Toronto.
The blackout triggered particular concern among New York residents
Gabriela Mira, 40, and her six-month-old daughter waited in a long queue to use a public telephone in New York.
"I had to get out of the house," she said. "It was so
dark, and everything was off, and I was scared. No air
conditioner, the phones - they need electricity. And it's
so, so hot."
Other affected cities include Detroit in Michigan as well as Cleveland and Toledo in Ohio and Toronto and Ottawa in Canada.
There were reports of outages in New Jersey and Connecticut as well.