NEW YORK (CNN) -- A major power outage simultaneously struck dozens of cities in the United States and Canada late Thursday afternoon.
Cities affected include New York; Cleveland, Ohio; Detroit, Michigan; Toronto and Ottawa, Canada. The power outage occurred shortly after 4 p.m. Officials said the outage does not appear to be related to terrorism.
By 6 p.m. the power was being restored in parts of the affected area, starting with the northern and western edges, New York Mayor Michael Bloomberg said at a news conference.
He said it was unknown how long the outage may last but did say it would be "hours not minutes."
New York Gov. George Pataki declared a state of emergency for the state and deployed additional state police.
State officials said the outage was likely a natural occurrence. They said the Niagara-Mohawk power grid, which provides power for New York and stretches into Canada, was overloaded.
Mayor Michael Bloomberg told a news conference there was "absolutely no indication" of terrorism.
Black smoke coming from a power station on East 14th Street was a natural reaction to the power company shutting down its boilers, Bloomberg said.
"No damage was done to the Con Ed facilities," Bloomberg told CNN.
A statement from the Department of Homeland Security said, "The department is working with state and local officials an the energy sector to determine the cause of the outage and what response may need to be taken."
Bryan Lee, a spokesman for the Federal Energy Regulatory Commission, said based on preliminary information it appears that a "cascading blackout" was caused by an outage at a Manhattan power plant, which has destabilized the power grid as far north as Canada and as far west as Detroit and Cleveland.
FERC and the Department of Energy were monitoring the situation and evaluating the problem, Lee said.
Frank McCarton, deputy commissioner for the Office of Emergency Management, said the blackout may have originated in Canada. "We understand that, from Con Ed, that we had a power surge and a dip and a failure that originated in Ontario," McCarton said.
Much of Midtown Manhattan and Wall Street was shut down. All area airports and the Long Island Railroad were also affected.
The Federal Aviation Administration grounded planes at all three New York area airports and in Cleveland because the blackout affected security screening facilities. The FAA reported planes also were grounded at the Toronto airport. The airports were operating on backup power, officials said.
The FAA said flights at the airports affected by the power outages were taking off, and flights already in the air were being allowed to land, with delays of no more than 15 minutes being reported.
The New York City Police Department said a number of people were trapped in elevators. Thousands of people could be seen leaving buildings and walking into the streets. New York subways were reported stopped and people were trapped in the cars.
"We are going to have a situation where people are going to have to walk a long distance. They need to be careful," Bloomberg said. "Our advice is to go home, open up your windows, drink a lot of liquids."