The largest power blackout in U.S. history rolled across a vast swath of the northern United States as well as southern Canada on Thursday, driving millions of people outdoors into stifling rush hour streets — then darkness.

New Yorkers escaped silenced subways. Nuclear power plants in four states shut down.

“We all are wondering what caused this,” said New York Gov. George Pataki. President Bush ruled out terrorism. The blackouts set off finger pointing on both sides of the border.

Canadian officials blamed the power outage on a lightning strike at an electrical plant near Niagara Falls. But officials at the New York Power Authority denied that lightning had struck the facility and said the plant was operating. Other U.S. officials said they were investigating a possibility that the blackout was caused by a transmission problem from Canada.

The blackouts started shortly after 4 p.m. EDT, engulfing most of New York state and parts of New England, and spreading west to Ohio and Michigan. In Toronto, Canada’s largest city, workers fled their buildings when the power went off. There also were widespread outages in Ottawa, the capital.

Power began to come back as evening wore on, but officials said full restoration would take much longer. Officials in Detroit urged people to stay home during the night; nearby communities declared curfews to keep problems to a minimum.

By Thursday night, New York authorities had electricity back on in parts of the Bronx, Westchester County and Long Island. About half of the one million homes and businesses that lost power in New Jersey had it back.

Outages ranged over an area with roughly 50 million people.

City dwellers cope

Cars try to navigate their way through New York City during a blackout that hit steamy U.S. and Canadian cities, stranding people in subways, closing nine nuclear power plants from New York to Michigan and choking streets with workers driven from stifling offices.

New Yorkers scrambled down endless stairways in skyscrapers where elevators stopped working, and some subway commuters were stuck for several hours underground. In the city that took the brunt of the Sept. 11, 2001, terrorist attacks, people filed into the streets with little fuss and looked for ways to get home.

“I’m trying to keep calm,” said Aaron David, 27, who works at the United Nations. “But I was here for 9-11. This doesn’t happen every day.”

Traffic lights were out throughout downtown Cleveland and other major cities, creating havoc at the beginning of rush hour. Cleveland officials said that without the power needed to pump water to 1.5 million people, water reserves were running low.

New York state lost 80 percent of its power, said Matthew Melewski, speaking for the New York Independent System Operator, which manages the state power grid. Both New York and New Jersey declared states of emergency.

Activities shut down

As darkness fell, city dwellers turned to candles and flashlights as scattered parts of the electrical grid came back on. People gobbled ice cream from street vendors before it melted, and gathered around battery-operated radios for updates.

Su Rya, 69, in batik shirt and shorts, guarded a store on 125th Street in Harlem. But when asked about talk that looting might break out, he said, “That’s barbershop talk. It’s a different generation now.”

Pedestrians and vehicles cross New York's Brooklyn Bridge after a massive power outage. Thursday's failure cut the electricity supply to millions of people in a 600-mile swath between New York and Detroit.

Marveled another man, “You can actually see the stars in New York City.”

There were outages in several Vermont towns and in northern New Jersey, where Gov. James E. McGreevey mobilized 700 National Guardsman and ordered 300 extra state troopers on duty. In Connecticut, Metro-North Railroad service was knocked out. Lights flickered at state government buildings in Hartford.

Broadway shut down. Night baseball, too.

The Mets were trickling out for batting practice in New York when the blackout hit and the game was canceled. Hours later, the visiting San Francisco Giants were still waiting in the parking lot for their bus. Some 500 miles west, the Toledo Mudhens’ International League game with the Norfolk Tides was called off, too, to be made up as part of a doubleheader tonight.

Varying reports

“We have been informed that lightning struck a power plant in the Niagara region on the U.S. side,” said Jim Munson, speaking for Canadian Prime Minister Jean Chretien.

But Brian Warner of the New York Power Authority said its Niagara facilities were not hit by lightning and “at no time during this incident ceased to operate.

In San Diego, Bush said “slowly but surely we’re coping with this massive, national problem,” and added that he would order a review of “why the cascade was so significant.”

Bush said he suspected that the nation’s electrical grid would need to be modernized.

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 Albany, N.Y., several people were trapped in elevators in Empire State Plaza, but most had been freed by 5 p.m. People in New York City lined up 10 deep or more at pay phones, with cell phone service disrupted in some areas. Times Square went dark.

Terrorism ruled out

In Cleveland, Olga Kropko, a University Hospitals labor and delivery nurse, said the hospital was using its backup 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.

The FBI and Homeland Security Department both said the outages appeared to be a natural occurrence and not the result of terrorism.

Police in Mansfield, Ohio, spread into the streets to keep traffic flowing. “A lot of officers are out there trying to make sure nobody gets hurt, to try to cut down on the accidents,” said jail officer Randi Allen.

Worst in history

The blackouts easily surpassed those in the West on Aug. 10, 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, 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.

New York City Mayor Michael 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.

As for the cause, he said: “It was probably a natural occurrence which disrupted the power system up there and it apparently for reasons we don’t know cascaded down through New York state over into Connecticut, as far south as New Jersey and as far west as Ohio.”

Security precautions

Nine nuclear power reactors — six in New York and one each in New Jersey, Ohio and Michigan — reported they were shut down because of the loss of offsite power, according to the Nuclear Regulatory Commission in Bethesda, Md.

The blackout set off security precautions developed after the World Trade Center attack, with heavily armed teams of counterterror officers deploying at New York City landmarks and other sensitive locations.

Officials swiftly realized the outage was not an act of terror and then used teams to make sure no one took advantage of the blackout to strike at a terror target, officials said.

Flights at six airports — Kennedy, LaGuardia, Newark, Cleveland, Toronto and Ottawa — were grounded, according to the U.S. Transportation Department.

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.”

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.