The outage affected a wide swath of territory in the U.S. and Canada - including New York City, Albany, Hartford, Toronto, Ottawa, Detroit, Cleveland and Ontario - and has officials in the two countries engaging in a blame game as to what went wrong.
In Cleveland, the loss of power also meant a loss of water - as there was no way to continue pumping water to 1.5 million people. The situation left the mayor there, Jane Campbell, angrily denouncing stores she said engaged in price gouging for water and other items, including batteries.
As dawn approached in the New York area, lights were reported flickering back on in Times Square, on Fifth Avenue, much of Staten Island, parts of Brooklyn and the Bronx, parts of Westchester County, N.Y., and parts of New Jersey and Connecticut.
But the New York metropolitan area is still in a major mess, with full power still not back, meaning that subway and train systems are also not back. Transit officials have said even when the power does come back, it will take as much as six hours for trains to start running normally.
But some folks will have to go to work anyway today. And despite today's 90 degree weather forecast, some are facing the inevitable: saddling up in the most comfortable socks and shoes they've got, heading out on the long walk to work.
One man started out across the Brooklyn bridge shortly after 5 a.m., on his way to his job at a Wall Street area gym, where he figures stranded New Yorkers are bound to take refuge hoping for a shower. Will the gym have water? He hopes so.
And will the opening bell ring on Wall Street as usual? The determination is certainly there: a convoy of emergency generators was spotted overnight on its way down to the financial district, which made a slew of backup plans after Sept. 11 and may today be putting those emergency strategies to the test.
Late Thursday night, before retiring for a short nap, New York City Mayor Michael Bloomberg said he expects "everything to be back to business" on Friday. But he also cautioned "I don't want anybody to think that the power is going to be back for everybody in the next hour. It is not going to be."
The power outage affected a broad swath of the Northeast stretching west to Ohio and Michigan and into southern Canada, starting shortly after 4 p.m. EDT. In Toronto, Canada's largest city, workers fled their buildings when the power went off. There also were widespread outages in Ottawa, the capital.
A power transmission problem from Canada was being looked at as the most likely cause for what some are calling the biggest electrical outage in U.S. history, said a spokeswoman for New York Gov. George Pataki.
However, Canadian authorities said it appeared lightning had struck a power plant on the U.S. side of the border in the Niagara Falls region, setting off outages that spread over an area of 9,300 square miles with a population of roughly 50 million people.
President Bush said Thursday evening that people affected by the huge blackout may not see their lives return to normal right away, but "slowly but surely we're coping with this massive, national problem."
The president told reporters in San Diego: "I have been working with federal officials to make sure the response to this situation was quick and thorough, and I believe it has been."
But, Mr. Bush said, state and local officials have not asked the federal government for much help as of yet.
Terrorism is not the cause of the outage, the president said.
Nine nuclear power reactors - six in New York and one each in New Jersey, Ohio and Michigan - were shut down because of the loss of offsite power, according to the Nuclear Regulatory Commission in Bethesda, Md.
Flights in and out of Kennedy Airport in New York, as well as airports in Toronto and Ottawa were grounded, leaving passengers stranded. Flights also were halted for more than three hours in and out of New York LaGuardia, Cleveland and Newark, N.J., but those airports had reopened by 8 p.m. EDT.
The blackout closed the Detroit-Windsor Tunnel, which 27,000 vehicles use daily, and silenced the gambling machines at Detroit's Greektown Casino. Patrons filed into the afternoon heat carrying cups of tokens.
Traffic lights were out throughout downtown Cleveland and other major cities, creating havoc at the beginning of rush hour.
Gov. Pataki said more than half of New York State is without power. He said there are supposed to be backup systems to prevent blackouts from snowballing, and that "there have to be some tough questions asked."
In New York City, subways and elevators lost electricity or resorted to limited backup power. Thousands of people streamed into the streets of lower Manhattan in 90-degree heat, and some subway commuters were still stuck underground hours after the blackout hit.
Amtrak suspended passenger rail service between New Haven, Conn., and Newark. Some northbound trains from Washington, a city that did not lose power, turned around at Newark.
There were outages in northern New Jersey and in several Vermont towns. Lights flickered at state government buildings in Hartford, Conn.
In Massachusetts, Kim Hicks of Baltic, Conn., was on the Cyclone roller coaster at a Six Flags amusement park in Agawam when the power stopped. ``We sat there about 20 minutes and they finally came to walk us off,'' she said. The park regained power a short time later.
In Cleveland, Olga Kropko, a University Hospitals labor and delivery nurse, said the hospital was using its back-up generators and had limited power. "Everyone is very hot because the air conditioning is off," she said. "Our laboring moms are suffering."
John Meehan, 56, walked down 37 stories in the BP Tower in downtown Cleveland, wearing his suit and carrying a briefcase. "It makes you wonder, was this terrorism or what?" he asked.
In Washington, the Health and Human Services Department said the biggest health concern was people getting overheated and dehydrated, something local health systems appeared to be handling, said spokesman Campbell Gardett.
The blackouts easily surpassed those in the West on Aug. 11, 1996, in terms of people affected. Then, heat, sagging power lines and unusually high demand for electricity caused an outage for 4 million customers in nine states.
An outage in New York City in 1977 left 9 million people without electricity for up to 25 hours. In 1965, about 25 million people across New York state and most of New England lost electricity for a day.
On Thursday, Mayor Bloomberg asked the city's more than 8 million people to be calm, go home, open windows and drink water.
"Be sure you don't make an inconvenience into a tragedy," he said.
New Yorkers - and tourists who got caught in the Big Apple when the power went out - took that advice to heart as they struggled through the darkness trying to get home.
Streets usually bathed in light and marquees were instead pitch black, seeming empty - although they weren't. Every few steps revealed someone walking or standing in the darkness, wondering what to do next.
Some people held blackout parties, in restaurants, and on sidewalks, gathered in circles around candles stuck in bottles.
For New York police, the focus was on the ramifications of the blackout rather than its cause.
"We're more concerned about getting the traffic lights running and making sure the city is OK than what caused it," said a spokesman at the department's operations center downtown.
"The good news is that in New York City, while we have lost all the power, Con Ed's facilities have shut down properly, which we have programmed them to do," said Bloomberg.
In Times Square, Giovanna Leonardo, 26, was waiting in a line of 200 people for a bus to Staten Island.
"I'm scared," she said. "It's that unknown 'What's going on?' feeling. Everyone's panicking. The city's shutting down."
Along several blocks near midtown Manhattan, deli owners brought their suddenly unrefrigerated food out on tables, iced in buckets. "Half price on everything," read one sign.