A fast-moving violent thunderstorm which may have spawned a tornado cut power to thousands of homes and businesses in southwestern and southern Ontario early Wednesday.

Environment Canada had issued a tornado warning for Hamilton at about 1:30 a.m., which prompted Emergency Management Ontario to issue a red alert for the area.

People were advised to immediately move indoors and away from doors and windows.

The storm packing winds of up to 90 kilometres an hour travelled from the Kincardine area in the north through the Hamilton and Niagara region.

Hamilton resident Wayne Scott was awoken by a large crash, and went outside to find a large spruce tree had split and hit his house.

"I was in a dead sleep, and it woke me right up. It was loud," he told CBC.

The tree poked through the siding and pierced the drywall near his bed, while the top of the tree caused damage to his roof.

Cleanup crews throughout the area had their hands full dealing with downed trees and broken hydro lines that left tens of thousands without power.

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High winds cause a spruce tree to split and fall into this house near Hamilton on Wednesday night. ((CBC))

Hydro One said Wednesday morning nearly 24,000 customers were without electricity, including more than 10,000 from Wiarton to Kincardine and east to Dundalk.

Another 6,800 homes and businesses in the Guelph area were also affected.

The fast-moving storm was almost all out of Ontario by about 3 a.m. and heading into the United States.

Environment Canada meteorologist Geoff Coulson surveyed the damage on Wednesday and said it was more consistent with a wind storm than with a tornado, which would produce a long, narrow damage track instead of a broad one.

However, confirming tornadoes also requires eye witness accounts, which was not possible as Wednesday's storm touched down in the middle of the night.

There were also unconfirmed reports on Wednesday afternoon of tornadoes touching down in Minden, Ont., which is located 100 kilometres north of Peterborough.