The largest blackout in U.S. history that struck America's Northeast corridor and parts of Canada Thursday afternoon affected transportation, utility companies, banking systems and scores of other corners of everyday life.

The outage left more than approximately 3,700 square miles of territory in darkness. Cities without power included: Erie, Pa.; Syracuse, N.Y.; cities in western Connecticut; Batavia, N.Y.; Detroit; New York City and surrounding areas; Ottawa; Toronto; Plattsburgh, N.Y.; Toledo, Ohio; Cleveland; Akron, Ohio; Canton, Ohio; and New Haven, Conn.

Thanks to the blackout, rush hour in major urban centers became even more horrendous. Traffic lights blinked out in Cleveland, New York and other U.S. cities while police tried to direct vehicles and the hordes of people walking home.

Thousands more were trapped underground in the dark when subway trains ground to a halt in their tunnels.

The New York Port Authority reported that buses, trains and other forms of transportation were stalled. Tens of thousands of people tried to line up at the ferryboat to get home to New Jersey.

Outbound bridges were open, but inbound bridges had been closed so as not to create more congestion.

Flights were halted for several hours in and out of Kennedy and LaGuardia airports in New York, as well as Cleveland, Newark, N.J., Toronto and Ottawa. All four U.S. airports reopened Thursday evening.

Other airports also were affected: Northwest Airlines curbed flights into Detroit and delayed, diverted or canceled flights in Hartford, Conn., and Lansing, Mich. US Airways reported problems in Albany and Rochester, N.Y., Erie, Pa., and Montreal. American Airlines had difficulties in Syracuse, Buffalo, Islip and White Plains, N.Y.

Anticipating big, ripple-effect delays as service was restored, several airlines were urging passengers scheduled to fly on Thursday to delay travel until later this week if possible.

Earlier Thursday American Airlines, the world's largest carrier, said all flights were stalled at 11 airports — in New York; Toronto; Cleveland; Detroit; Newark, N.J; Ottawa; Syracuse, N.Y.; Buffalo, N.Y., and Montreal.

"All flights on the ground at time of outage will remain on the ground until power is restored," an American Airlines spokesman said.

Amtrak suspended passenger rail service between New Haven, Conn., and Newark, N.J. Some northbound trains from Washington, D.C., which didn't lose power, turned around at Newark.

Delays were expected to extend into Friday as the airlines and Amtrak worked through the night to get planes and trains into position to handle the backlog. The FAA, the airlines and Amtrak urged passengers to check before heading to the airport or train station.

"You're going to get some residual delays as aircraft are put in position, the effects being no more than after a severe snowstorm or severe thunderstorms," Federal Aviation Administration spokesman Greg Martin said. "It will take into the morning and perhaps to midday to get the system in place."

Air traffic came to a halt at the six airports Thursday because there was no power to run the metal detectors and X-ray machines at security screening checkpoints, Transportation Department spokesman Leonardo Alcivar said. In addition, flights to those airports were not allowed to take off from their originating airports.

Planes already in the air were allowed to land at the airports, directed by air traffic controllers operating with emergency power. That also allowed controllers to continue to direct flights to and from other airports, minimizing delays, Alcivar said.

The blackout provided yet more troubles for the struggling airline industry, which is still trying to recover from the Sept. 11 terrorist attacks.

"We're a resilient bunch," said Jim May, president of the Air Transport Association, the major airlines' trade group. "At the moment, we'll play with the cards we're dealt."

New York state lost 80 percent of its power, leaving over half of the state's 19 million residents in the dark. Both New York and New Jersey have declared states of emergency.

"There is no evidence whatsoever of terrorism," New York City Mayor Michael Bloomberg said during a press conference. "Everything is calm in the city," he added, saying power was expected to return sporadically over the next few hours.

There were outages in northern New Jersey and in several Vermont towns. In Connecticut, Metro-North Railroad service was knocked out. Lights flickered at state government buildings in Hartford, Conn.

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.

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."

Lack of power has caused all four water plants serving the Cleveland area to shut down. Officials say they have about two hours of water in reserve. State administrators are trying to figure out how to handle the water shortage, WJW Fox 8 reported.

With no power in downtown Cleveland, the city's Mayor Jane Campbell has declared a curfew mandating that kids 18 and under must be indoors after 9:30 p.m. ET.

Eight counties in southeast and central Michigan were hit by the blackouts, leaving two million people in the dark. State officials are asking residents to conserve water in areas where electric pumps are the primary source of water delivery.

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.

In Detroit, there are reports of scattered looting, accidents and traffic backups. The Detroit Windsor Tunnel and the Ambassador Bridge, which both lead to Canada, were both reportedly closed.

Detroit police in the suburbs say they have declared a state of emergency. Detroit has also initiated an emergency curfew.

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.

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. Backup diesel generators are currently running and have the capability to run for several days if necessary.

When an instability or problem occurs in the power grid, plants immediately go into shutdown mode for safety reasons.

The American Stock Exchange and Nasdaq reported "minimal interruption" after the close of trading. Both had backup power generators. The New York Stock Exchange said it was too early to know whether trade processing was affected by the outage. All three exchanges said they didn't know whether the outage would interrupt Friday trading.

The blackout fueled investors' fears that Friday could be a rough trading day on Wall Street with concerns over business disruptions in the affected area. An NYSE representative told Fox News that they are preparing for a 9:30 a.m. regular opening for Friday's trading session, even if it has to operate on a generator.

The Nasdaq 100 fell 6.00 points to 1,239.50 and the Standard & Poor's 500 index shed 4.70 points to end the day at 983.00.

The nation's largest phone company, Verizon Communications, said backup generators worked quickly and that land-line service was uninterrupted. Cell-phone service was lost temporarily because the network was overloaded as people around New York tried to use their phones.

Citigroup invoked its "business continuity" plans. General Motors said multiple auto plants were affected.

In Canada, as many as 10 million people were affected by the outages, which brought most of southern Ontario to a halt.

In Toronto, streetcars preparing to transport workers around downtown for the evening rush hour ground to a halt, sending riders into the street to hail taxicabs. "The streetcar can't go anywhere, you just have to wait," said Mike Collins, a streetcar driver with the Toronto Transit Commission.

Both the Toronto Stock Exchange -- the country's main bourse -- and Pearson International Airport were operating on backup power supplies, Reuters reported.

Power was still on in Montreal, Quebec City and most of Quebec. A spokesman for Montreal's Dorval airport told Reuters that all flights to blackout cities have been canceled, including Toronto.

Government sources also said the Canadian and U.S. military air patrols that watch the skies over the Northeast beefed up patrols.

The Associated Press contributed to this report.