MONTREAL — Repair crews are working around the clock to restore electricity to about 500,000 households in Ontario and Quebec after an ice storm snapped branches, brought down power lines and crimped travel in Canada’s two most populous provinces.

In Toronto, Canada’s biggest city, about 244,000 Toronto Hydro customers were without power as of 3:15 a.m. Monday, the utility said in a message on Twitter. All streetcar service in the city — on what would have been one of the year’s busiest shopping days — was suspended Sunday due to icy power lines, the Toronto Transit Commission said. Street car service was expected to be restored this morning, while the Sheppard and Scarborough RT subway lines remain closed, the commission said.

“This is truly one of the worst ice storms we’ve seen here in Ontario,” Toronto Hydro Chief Executive Officer Anthony Haines said Sunday at a news briefing. While all available employees have been deployed, service may not be restored in full before Tuesday, Christmas Eve, the utility said.

More than 400 flights were canceled Sunday at Toronto’s Pearson International Airport, according to the airport’s website, with dozens more scrapped in Ottawa and Montreal. The storm left up to 1.2 inches of ice in the Toronto area and may dump as much as .4 inch in southeastern Quebec Monday, Environment Canada said.

Toronto’s East General and Sunnybrook hospitals are operating on emergency generators, as is the city’s water- pumping system, Haines said.

“The top priority now is the hospitals,” Toronto Mayor Rob Ford said at a news conference at City Hall.

Provincial officials, including those from Emergency Management Ontario, are working with affected municipalities to ensure a coordinated response, Ontario Premier Kathleen Wynne said at a separate news conference Sunday. The Ontario government will provide tree harvesters to cities that have requested help, she said.

“I want to assure everyone living in these areas that all available resources are working to keep you and your family safe, and to restore power as quickly as possible,” Wynne said.

Ford said it’s too early to declare a state of emergency.

“If it gets really bad in the next 24 hours we could have a state of emergency but I don’t want to say that right now,” he said. “We’re not in that situation quite yet.”

Ice is building up on some transformers, which could trigger “catastrophic” equipment failures, Haines said.

“It’s not just a matter of going in and restoring the power lines,” he said. “Now we’re going to be replacing poles, replacing transformers at the top of some of these poles, so it’s going to be a major event that is going to last days for us to be able to get the power back up.”

Hydro One, another Toronto-based utility, said in a statement Sunday that about 120,000 customers had no power.

“Crews are finding tree branches and power lines coated with more than an inch of ice, so restoring power is slow going,” said Greg Towns, Hydro One’s director of lines.

PowerStream, a utility in the York region north of the city’s downtown core, said about 57,000 customers were without power. Enersource, which is based in the city of Mississauga, had about 2,800 clients affected by the outages, while Horizon Utilities, in Hamilton, had about 30,000. PowerStream said service will probably be restored within 48 hours, while Horizon said outages may last as long as 72 hours.

In Quebec, about 51,000 Hydro-Quebec customers were without electricity, the Montreal-based company said on its website. Icy conditions may have played a role in at least three fatal accidents on Quebec roads, Canadian Press news agency reported.

Further east, about 4,500 customers in the province of New Brunswick were without power, New Brunswick Power said.

Air Canada, the country’s biggest airline, said Sunday it would waive fees to allow passengers to change flights, space permitting, as the storm affects operations. Via Rail, the country’s passenger rail operator, said it’s not expecting any cancellations, though delays were likely.