The North American power blackout has brought most of southern Ontario to a standstill as up to 10 million people are left without electricity.
Cause of the outage is not yet known, though there have been indications it may have originated in the Niagara Falls region.
In Toronto, Canada's largest city, the transit system ground to a halt, and thousands were stranded as temperatures hit 30 degrees.
Transit authorities shut the doors into subway stations to prevent overcrowding and an officiald said people were evacuated from underground.
The blackout hit just as commuters headed home and that left thousands milling about downtown streets. A witness described the scene as "pretty chaotic."
A spokesman in Toronto Mayor Mel Lastman's office said the city is setting up an emergency command centre."We're still gathering information at this point, but there's 2.5 million people that have to get home from work," the spokesman said.
Both the Toronto Stock Exchange, the country's main bourse, and Pearson International Airport were operating on back-up power supplies.
Power was still on in Montreal and most of Quebec. A spokesman for Montreal's Dorval airport said all flights to blackout cities have been cancelled, including Toronto.
The power outage, which happened minutes after 4 p.m. (9 p.m. Irish time), also hit the Canadian capital, Ottawa.
The massive power outage also swept across swaths of the eastern United States, leaving sections of New York City, Detroit and Cleveland without electricity.