A spokeswoman in the office of Canadian Prime Minister Jean Chretien said on Thursday night that lightning hit a power plant in New York state and started a cascading blackout over an area of 9,300 sq. km in northeast United States and Ontario.
Officials on both sides of the US-Canada border have confirmed this, the spokeswoman said.
"It cascaded to Chicago, New York City, Toronto, Boston and North Bay, Ontario," the spokeswoman said, adding that a fire erupted at the power plant after the lightning.
She also said the main Ontario power utility was doing all it could to "decouple" itself from the US power grid, to which it is interconnected.
"We have no estimates on the expected length of the blackout," she said from Ottawa.
The power outages were reported starting shortly after 4:00 p.m. EDT (2000 GMT).
Subways in New York came to a complete halt, leaving thousands of rush-hour commuters stranded underground in the dark. Traffic was at a standstill throughout the city and sidewalks were jammed as commuters headed home on foot.
Power was out at all three of the New York area's major airports. There were no buses or commuter trains running, and cellular telephone service was disrupted.
"Everything is calm in this city. The New York City power grid was not damaged when it shut down, as it should," Mayor Michael Bloomberg said.
The outage struck nerves among New Yorkers, whose memories of the September 11 attacks still run strong, and people could be seen running through the streets of the city's downtown financial district toward the East River and Brooklyn Bridge.
"Everything is calm in this city. The New York City power grid was not damaged when it shut down, as it should."
--Mayor Michael Bloomberg
"Scared," said Jeffrey Snop, of Queens, at the Times Square subway station. "It reminded me of 9/11 and stuff like that."
Bloomberg urged calm and said the greatest risk to New Yorkers was from overheating without air conditioning.
The high temperature in New York City on Thursday was 33 degrees Celsius, reported a few hours before the outage occurred.
Times Square was in a state of confusion. The traffic lights were out, and police were directing the rush-hour flow on what is typically one of the most congested spots in the city.
The power outage hit much of the Canadian province of Ontario, shutting major cities including the Canadian capital Ottawa and Toronto.
In Toronto, Canada's largest city, the transit system ground to a halt. But both the Toronto Stock Exchange, the country's main bourse, and Lester Pearson Airport were operating on back-up power supplies.
In Detroit, headquarters to the largest US automakers, many workers decided to go home after the lights suddenly went out, creating traffic gridlock in many streets and highways from the city core.
Chaos in New York City
Large parts of the city including the downtown headquarters of General Motors Corp appeared to be affected. The outage came on a day predicted to be one of the hottest of the summer.
Media reports said an overload of the Niagara Mohawk power grid and outages had occurred as far away as Cleveland, Ohio.
The Oyster Creek nuclear power plant in Forked River, New Jersey was apparently not functioning either.