Hundreds of thousands of Canadians face the prospect of a Christmas in the dark, as power crews warn it could take days to restore service in the aftermath of an ice storm that knocked down power lines across several provinces.

By Tuesday morning, about 126,000 homes were still without power in Toronto, with Toronto Hydro able to restore power to 57 per cent of affected homes overnight.

About 385,000 homes, businesses and other buildings were without power Monday evening across Ontario, Quebec, New Brunswick and Nova Scotia. About 195,000 of the buildings were in the City of Toronto, where one official estimated about half a million people were without power and warned it could take days to reconnect every home.

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"The prudent thing to do now is to plan for the worst and hope for the best. So people need to make alternate arrangements," Toronto Hydro CEO Anthony Haines said Monday, adding: "It would be irresponsible to say, 'Well, by Christmas morning, we'll all be back [online].' That's simply a promise we can't give at this point."

While Ontario's cleanup continued, New Brunswick took the brunt of the storm's force on Monday. The province began the day with 7,000 outages and had 50,000 by evening. New Brunswick Power hoped to reconnect most by late Tuesday, but others could remain without power through Christmas. "There are others that we're just going to have to see how it goes," spokeswoman Deborah Nobes said.

Hydro Quebec had 40,000 clients without power as of Monday evening, while Nova Scotia power had about 11,000 clients without power. The numbers represent the number of buildings affected, while the number of people is typically more than twice as high, Mr. Haines said.

Southern Ontario appeared worst hit. "Ice has built up on trees, and trees have fallen down on wires. That's really the sum of it," Mr. Haines said. Officials opened "warming centres" throughout Toronto, for those without power as temperatures were set to dip to 11 degrees below freezing overnight. Premier Kathleen Wynne called on those who have electricity to invite in friends and neighbours who don't. "There is progress being made, but there is still need," she said.

Toronto Mayor Rob Ford said the worst of the weather appears to be over. "We're going to stay here every day, even Christmas Day, until every light is back on in the city," he said.

Power lines, however, were still at risk in the region. "The ice has not melted, it's on those trees, and any sort of drop in temperature and increase in wind activity will mean those branches will come down," said Carmine Marcello, CEO of Hydro One, which serves much of Southern Ontario.

Officials played down questions about why Toronto hadn't declared a state of emergency. Mr. Ford said the damage didn't warrant it. He has had some of his powers stripped after recent revelations about drug use, and some councillors questioned whether politics played a role in refusing to declare a state of emergency.

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Regardless, Mr. Haines said Toronto Hydro was already responding with all hands on deck. "I feel comfortable saying there would be no change in the operation of Toronto Hydro, regardless of the operation of the city's emergency management office," he said.

Crews were being dispatched from Manitoba, Michigan, Windsor, Mississauga and Ottawa to help restore power in Toronto. After restoring power to hospitals, a water treatment plant and most of the transit system, Toronto Hydro was turning its attention Monday evening to repairing large "feeder" electrical lines. After those are online, crews will begin the arduous task of connecting individual homes. "We're expecting a lot of hard work ahead of us," Mr. Haines said.

Ontario's Associate Chief Medical Officer of Health Robin Williams said there had been a "surge in attendance" in Toronto's emergency rooms, due in part to people falling on ice, and she called on people to "be respectful of using the emergency department as you need to."

After the 1998 ice storm that hit Ontario and Quebec, power companies reinforced their infrastructure, trimmed back trees and struck deals with nearby utilities to prepare for the worst. But the industry has warned that major overhauls of the grid are costly, and would likely boost power rates.

The economic effect of the storm isn't clear. In a note sent out to clients, CIBC World Markets said the 1998 ice storm led the GDP to drop, only to see it rebound afterward, and that it expected this year's storm to have a "negligible" impact. "We do not view this storm as a major economic event," CIBC said.

With reports from Bill Curry and Rob Carrick