The storms produced strong winds, hail and several tornadoes, forecasters said.
A 21-year-old Montreal woman was killed when she was struck by a falling tree branch. A boy, 6, and another woman were injured.
Rainwater systems were overloaded when more than an inch of rain fell in just 15 minutes, leaving roads flooded throughout Montreal, the Canadian Broadcasting Corp. said.
At the peak of the power outage, 400,000 people in Quebec and 108,000 people in Ontario were without electricity Friday night.
Hydro-Quebec said 250,000 people remained without power at midday Saturday.