Seven people were killed by falling trees and a woman died when a boat capsized in the Ottawa River.
Wind gusts reached 82 mph (132km/h) during the storm, according to Environment Canada.
Power company Hydro One, which covers Ontario, said it would take several days to reconnect every home.
Meanwhile, Hydro Quebec said 550,000 homes there lost power, with nearly 400,000 still suffering outages as of 10:00 local time (14:00 GMT) on Sunday.
Prime Minister Justin Trudeau tweeted on Sunday evening that the federal government was preparing to step in to help those affected.
“The storms that swept across Ontario and Quebec yesterday caused serious damage, claimed several lives, and left many without power,” Mr Trudeau said.
“We’re thinking of everyone affected, and thanking the crews who are working to restore power – we stand ready to provide federal support if needed.”
Pictures on Canadian media showed firefighters and utility workers trying to untangle downed powerlines and poles that are lying over cars on a road in Ottawa.
The town of Uxbridge, about an hour north of Toronto, has declared a state of emergency after the storm left “significant damage in its wake”, a statement said.
The powerful thunderstorm, which lasted more than two hours, felled many trees, disrupted traffic, damaged homes and saw emergency services inundated with calls for help.
Experts said the storm, which spanned across an area of around 621 miles (1,000km), was caused by a rare phenomenon called “derecho”.
Derenchos are caused when a fast moving group of thunderstorms band together and mostly occur during summer months.
Uprooted and overturned
The seven people killed by falling trees and branches were in the eastern Ontario province.
Provincial Police said a 59-year-old man was killed when a tree was uprooted on a golf course in the capital, Ottawa. East of the city, local media reported that a 44-year-old man was killed in Greater Madawaska.
One person died and two others were injured by an uprooted tree at a campsite about 60 miles (100km) from Toronto, and a woman in her 70s was killed while out walking in the suburb of Brampton, in the Greater Toronto area.
In neighbouring province Quebec, police told local media that a 51-year-old woman drowned when a boat overturned in the Ottawa River, which runs through both provinces.