At its peak, more than half-a-million customers in Quebec were without power after the storm on Friday. That number dropped on Saturday and was down to about 236,000 by late afternoon.
Hydro Quebec said most of the area around Montreal would likely have power back by the end of the weekend.
Other areas, such as the City of Laval and regions further north, could have to wait considerably longer.
It could be days before crews can access downed power lines in more remote areas because roads are blocked by fallen trees, said a spokesman for Hydro Quebec.
"It was the severity of the thunderstorms and the fact that it touched a whole lot of regions across Quebec that really made this exceptional," Gary Sutherland said in an interview.
Sutherland said workers were recalled from Quebec's traditional two-week construction holiday, which began Friday, to deal with the damage.
"It really swept from west to east," he said of the storm. "In some cases, trees were actually uprooted and then fell on the lines."
High winds accompanying the thunderstorms also damaged roofs and overturned cars, with gusts in the 100-kilometre-per-hour range in some places.
A municipal employee in Boucherville, a suburb of Montreal, died when a branch fell in a local park. Two other people were injured.
Eight children were injured in Prevost when a tent at a day camp came down on its occupants.
Areas north of Montreal, including Laval, the Laurentians, Lanaudiere, along with the Outaouais region were hit hardest by the storm. Portions of the Saguenay region were also hit on Saturday, leading to new outtages.
Sgt. Frederic Jean, a spokesman for Laval police, said they were flooded with a huge number of emergency calls during Friday's storm. Some power lines and trees were still down on Saturday and remained a safety concern, he said.
"I've been on the force for 14 years and I've never seen anything like it," he said.
In Ontario, some of the most affected areas included Pembroke and Petawawa on the Ottawa River, and Callander outside North Bay.
Hydro One reported 93,000 customers were without power by Saturday afternoon.
Environment Canada attributed the damage to a "vigorous cold front" out of the northwest.
The government agency said in a statement there were reports of significant hail, structural damage, funnel clouds and damaging winds.
It has dispatched teams to investigate the storm damage.
The storm even took out a piece of Canadian history.
In Toronto, a tree said to have inspired the song The Maple Leaf Forever was brought down.
The aged silver maple was among the many trees that fell as fierce winds moved through the region Friday night.
A nearby plaque says the tree's falling leaves inspired Alexander Muir, a school principal, to write the Maple Leaf Forever in 1867, the year of Confederation.
The powerful storm was the second to blast through Montreal in less than a week.
On Wednesday, about 60,000 Hydro Quebec customers in and around the city lost power after a brief but intense thunderstorm.
- with files from Helen Moka
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