Electricity is slowly being restored to parts of the eastern United States and Canada following the biggest power failure in US history.
Stranded commuters spent the night asleep on the street
About 50 million people were affected by the outage which crippled cities from New York to Detroit, and Toronto to Ottawa.
Thousands of commuters in New York were forced to spend the night sleeping on the streets after the transport network ground to a halt and a breakdown in the traffic signalling system caused gridlock on the roads.
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 insisted terrorism was not to blame.
A former US energy secretary, Bill Richardson, said the problem was lack of transmission capacity, describing the United States as a superpower with a third world electricity grid.
The blackout occurred on one of the hottest days of the year, when the demand for air conditioning soared.
In other developments:
- 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.
- Sporadic looting was reported in at least four areas of the Canadian capital, Ottawa.
- Twenty-six people were arrested for looting in Brooklyn, New York, Reuters news agency quoted police as saying
- The blackout closed the Detroit-Windsor Tunnel, which links the United States and Canada and is used by 27,000 vehicles daily.
The BBC's John Terrett in New York says the city woke up to an unusual sight - thousands of people lying on the pavements, propped up against shop fronts, lounging on the edge of fountains, huddled round small clusters of night lights chatting.
What transport is available, like city buses, is already clogged and there are still virtually no traffic signals working.
The authorities promised to restore power for the morning rush hour but while electricity is returning slowly to the suburbs, the island of Manhattan remains largely in the dark.
The New York Times website reported on Friday morning that power had also been restored to areas of the south-east Bronx, parts of Manhattan south of Central Park, western parts of Brooklyn, north-western Westchester County, western Queens, most of Staten Island and parts of New Jersey.
But New York Mayor Michael Bloomberg warned that full restoration would "take a decent amount of time".
Some rural areas in Michigan were reconnected, but authorities warned more populated urban centres would be powerless into the weekend.
Electricity has also been restored to western and central Toronto and about half of Ottawa, although officials there have warned of more blackouts during the day.
The power failure trapped thousands of people in subways and triggered some panic on the streets.
"Everybody just flipped out," said nurse Mary Horan, outside Grand Central Station.
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
"Suddenly you start thinking about 11 September."
As dusk faded into night, New York's famed skyline was shrouded in darkness and residents turned to candles and torches for light.
Times Square was plunged into darkness as giant billboards and television screens went black, while the lights also went out on Broadway, forcing theatres to cancel shows.
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.
Mayor Bloomberg told New Yorkers to treat Friday "as a snow day. It wouldn't be the worst thing to take a day off".
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, although the New York Times said a woman died before an ambulance could reach her amid the chaos.