DURHAM, Ont. - One person was killed as a violent storm harbouring suspected tornadoes tore across Ontario on Thursday, flattening buildings, tearing the roofs off homes and felling trees and power lines across a large swath of the province.

The intense storm prompted tornado warnings as it tracked east, and more than 69,000 Hydro One customers were without power in the aftermath.

The first reports of funnel clouds came from the southwestern Ontario community of Durham, where the West Grey Police Services confirmed one person was dead.

While police provided few details, Durham resident Gord Becker said a person had been killed in the conservation park towards the north of the town. Becker said people in the park had told him a workshed had collapsed in the storm, crushing a person inside.

There were unconfirmed reports that the victim was an 11-year-old boy.

"It's complete destruction," said Becker, adding that a number of buildings had been levelled as the storm roared through.

"The sky was just black, the clouds were moving in circles."

"There was tremendous amounts of tree damage, wires were down, I just barely got through the road."

The town was swarming with emergency personnel who were working frantically to attend to damages on Thursday evening.

Suspected tornadoes were also reported in two communities north of Toronto - Vaughan and Newmarket.

The intense storm had Environment Canada advising people to take shelter in basements or other secure areas, and at one point Toronto and Hamilton, among other large urban centres, were under tornado warnings.

Numerous flights at Pearson International Airport had been cancelled or delayed.

The sudden severe weather was thought to be the result of the sweltering temperatures of the past few days.

"There's some very warm and humid air acting as the fuel that's driving this thunderstorm development," said Environment Canada Meteorologist Geoff Coulson.

"Now it's just a question of trying to figure out how much damage it caused, its path and rating."

Residents in Vaughan reported roofs torn off homes and businesses.

"I saw three little small funnel clouds join themselves into one and just kept on coming towards my store," said Vince Varano, manager of the Highland Farms grocery store.

"I was just going to come to my store and lock everybody in."

Although his store escaped unharmed, Varano said he had heard of a grocery store further away which had its roof torn off.

There were also reports of cars being flipped over in a shopping centre's parking lot.

Debris scattered across the parking lot of another nearby mall, including concrete blocks and a satellite dish. Cars were displaced from their parking spots, streetlights were out and signs were broken.

"This is the air conditioning unit we had on our roof," said Robin Yadranji, pointing to a square scrap of metal in the parking lot of his car dealership.

"It landed on top of the cars, then they just twisted, went around and all the cars got damaged."

One business owner, who would only identify herself as Carm, said she was inside when she started seeing "things flying in the air and cars all over the place."

"It was pretty bad. You could see wind tunnels basically, and you could see things flying away with it," she said.

"And it happened it a minute."

Every available officer was brought in to deal with the storm aftermath in Vaughan, a York region police spokeswoman said.

There was widespread damage, power outages and debris, but police knew of no serious injuries, she said.

Police were appealing to residents to stay off the roads because of the dangers posed by debris and downed power lines.

There were also no reports of serious injuries after a possible tornado sighting near Newmarket. The suspected funnel cloud was spotted near an equestrian facility, police said.

Pat Zolnai, who lives in Durham, said the weather turned nasty mid-afternoon.

Zolnai said her next door neighbour had been driving home from the nearby community Mount Forest and had seen what she thought was a tornado touching down around 3:30 p.m.

"She saw a funnel," said Zolnai.

Mel Graham at the Leeson True Value Hardware store in the village of Markdale, near Durham, said although he didn't see a tornado, the heavy storm that swept through the town cut down power lines, causing a temporary blackout.

Graham said firefighters were walking the streets telling business owners and residents to return home and stay indoors.

-With files from Romina Maurino, Diana Mehta, Brian Pardoe and Peter Cameron