More than a million Canadians were without electricity on Thursday as an ice storm lashed the east of the country, knocking over trees and damaging power lines, a utilities provider said.

The environment ministry issued a freezing rain warning for Quebec province, with authorities warning that the buildup of ice "was heavy enough in some places to uproot entire trees."

As of 12.30am (0430 GMT Thursday), more than a million Hydro-Quebec customers were without electricity, the company said on its website.

"The outages were primarily caused by the weight of the ice which leads branches and trees to break and come into contact with power lines," Hydro-Quebec tweeted.

About half a million of those without power were in Montreal, where city crews worked to clear roads of broken branches and downed wires.

Montreal police received a "high volume of calls related to the ice storm" while city mayor Valerie Plante called for vigilance.

In neighbouring Ontario province, more than 100,000 customers lost power, with "high winds, lightning strikes and the accretion of freezing rain" damaging equipment and felling trees, said electricity provider Hydro One.