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It is thought to be the worst power cut in US history, and has affected more than 50 million people.
Traffic lights failed, underground railways were evacuated and people were trapped in lifts in offices and apartments. Flights into six airports in the affected areas were stopped for several hours.
There were extraordinary scenes in New York City as workers walked home, choking pavements and roads.
Civilians manned intersections directing gridlocked traffic after traffic signals failed.
Thousands of commuters were unable to get home and spent the night sleeping on the streets.
In many areas, the main difficulty was the soaring heat. The blackout happened on one of the hottest days of the year, with temperatures of up to 32C.
Manhattan worker Lucy was one of those who walked home, leaving her office at 1630 (1200 GMT) and arriving at her Jersey City home at midnight.
"I tried to get a ferry from NYC, but they soon stopped from the pier I walked to as they all ran out of fuel," she said.
"The queues were terrible - people fainting everywhere from the heat."
For many, the incident brought back memories of the terrorist attacks of 11 September.
Jessica Nottes was on top of the Empire State building when the power went out.
"We had to walk down 86 flights of stairs," she said. "I kept thinking about the Twin Towers and how I would get down. But everybody was calm."
President George W Bush was quick to give reassurances that terrorism was not to blame, and added, "We're slowly but surely coping with this massive national problem."
Canadian officials have pointed to a fire in a power plant near the upstate New York town of Niagara as the cause, although the United States disputes this.
A spokeswoman for New York Governor George Pataki said US officials were looking at a power transmission problem from Canada as the most likely source of the power cuts.
Work has been going on through the night to restore electricity to the worst-hit areas, and some power is returning although officials say restoring levels to normal could take much longer.
Power was up and running to nearly all areas after nearly 30 hours, although it took well into the next week for full service to be restored.
A report by a joint US-Canadian taskforce, published in November 2003, laid most of the blame for the blackouts on an Ohio-based plant operator, FirstEnergy.
It found one of FirstEnergy's plants had shut down unexpectedly, cutting off a major supply route into the main electrical grid.
The plant did not opt out of the electrical grid because its alarm system failed, leaving employees unaware of the problem.
The massive extra demands on neighbouring power grids led to a domino effect as successive states' power supplies became overloaded and failed.
There was also widespread criticism for the United States' antiquated power grid system, with President Bush calling the blackouts a "wake-up call" to do something about poor investment in power stations.
But a draft energy bill, including proposals aimed at preventing similar blackouts in the future, became stalled in the US Senate in November 2003 amid accusations of vested interests.
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