A major power outage plunged many American and Canadian cities into darkness at around 4pm (US time) on Thursday.

Cities affected include New York; Cleveland, Ohio; Detroit, Michigan; Toronto and Ottawa, Canada. Officials said the outage was not related to terrorism.

By around 6pm the electricity was being restored in many of the affected areas, starting with the northern and western edges, New York Mayor Michael Bloomberg said at a news conference.

"We will be starting up power in the city. It will take a decent amount of time -- hours, not minutes, and nobody really can be any more specific than that," he said.

The mayor mobilised 40,000 police officers and the entire fire department to maintain order. No reports of looting or other disturbances had been reported.

New York Governor George Pataki declared a state of emergency and deployed additional police.

According to reports, the blackout happened because the Niagara-Mohawk power grid, which provides power for New York and stretches into Canada, got overloaded.