A major power outage struck eastern North America Thursday afternoon, blacking out most of Ontario and a number of U.S. cities.

Front and Bay streets, 6 p.m.

Waiting for the bus on Yonge Street.

Yonge Street, empty.

The lights were out at Dundas Square.

Citizens help others and direct traffic.

Rush-hour traffic snarled in Toronto, Ottawa and other cities throughout southern Ontario as traffic lights went dark and transit systems ground to a halt at about 4:15 p.m.

There was no immediate explanation why the power went out. One report said the blackout was a result of a grid overload in the Niagara area.

The blackout extended into the United States, hitting such centres as New York City, Detroit and Cleveland.

Power returned in some southern Ontario communities, including Grimsby and Niagara-on-the-Lake, before 7 p.m.

There were reports of lights coming back on in some Toronto neighbourhoods at around 10:30 p.m., and many areas of the city had power restored by 11:15 p.m.

The city of Toronto had asked residents to use water sparingly while the blackout continued, saying its emergency supply was only expected to last for 24 hours

Police also asked residents to refrain from calling 911 except for real emergencies.

City streets in Toronto were initially clogged, as rush-hour motorists struggled through downtown intersections without the help of traffic lights.

Subways and streetcars were left paralyzed by the outage, but the Toronto Transit Commission continued to run buses throughout the evening.

Premier Ernie Eves declared a state of emergency across the province late Thursday, and asked any non-essential or non-emergency service workers to stay at home Friday.

Pearson International Airport suspended all flights out of Toronto, but was allowing incoming flights to land Thursday afternoon.

Peter Gregg, spokesman for the airport, told CBC News that the airport was safe, with enough backup power to handle emergency systems and air traffic control.

Sunnybrook Health Sciences Centre, North York General, St. Michael's Hospital and St. Joseph's Health Centre were all running on backup generators.

Adding to the misery was a heat wave that blanketed much of eastern North America.

Temperatures on Thursday were near 30 C in many areas hit by the blackout.