An angry black curtain of cloud carved a sudden and destructive swath across southern Ontario on Thursday, killing at least one person as a series of suspected tornadoes flattened buildings, stripped roofs off homes and toppled trees and power lines.

In the tiny town of Durham, Ont., south of Owen Sound, police confirmed a lone fatality but refused to comment on reports the victim was an 11-year-old boy. Witnesses said the person was inside a workshed when it collapsed.

"It's complete destruction," said resident Gord Becker, who described how several buildings were levelled by the storm as a low, rotating bank of cloud roared its way through town.

"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 alive with emergency personnel as crews worked frantically to attend to the damage and search for possible victims.

The intense storm prompted tornado warnings as it tracked east, leaving more than 69,000 Hydro One customers without power in the aftermath. Suspected tornadoes were also reported in two communities north of Toronto - Vaughan and Newmarket - and in Collingwood.

So severe was the threat that at one point, Environment Canada took the unusual step of advising people to take shelter in basements or other secure areas.

In Toronto, there were reports of a funnel cloud being sighted near the busy central intersection of Yonge and Bloor. Motorists tried to negotiate debris-strewn roads awash in ponds of heavy rain, many of them dark and without functioning traffic lights.

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

Prime Minister Stephen Harper, currently on a tour of Canada's North, expressed his concern.

"We share our thoughts and prayers with any who have lost lives or have dislocation because of this," Harper said. "Obviously the government of Canada, where appropriate, will work with local and provincial authorities to deal with this situation."

The storm, which is just the latest instance of severe, spectacular weather in the area in recent days, followed several days of sweltering heat and oppressive humidity - a regular feature of life in southern Ontario in the dog days of August.

Environment Canada said it considered the events of the day the worst it had seen in years.

"From year to year we do tend to get at least one big defining event and it does appear that the events of Aug. 20 will be one of these large scale events," said meteorologist Geoff Coulson.

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

Teams will be going out to Durham and areas north of the Toronto area over the next few days to assess the extent of the damage and rate the strength of the suspected tornadoes.

Residents in the suburb of Vaughan, north of Toronto, reported seeing the roofs torn from homes and businesses.

The city declared a state of emergency, with reports saying 120 homes had been evacuated.

"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."

Varano's store escaped harm, but another one not far away lost its roof. There were also reports of cars being flipped over in a shopping centre's parking lot.

Debris was 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 York region police officer was brought in to deal with the aftermath, a spokeswoman said. There was widespread damage, power outages and debris, but police were aware of no serious injuries, she added.

Mike De Pinto was driving on Highway 400 north of Toronto when he encountered the storm.

"You just see these clouds close to the ground and then they start spinning. It was raining a lot, everything was horrible," he said.

"It got to the point where you couldn't see four feet in front of you."

Once home in Vaughan he found one neighbourhood had been closed off because of gas leaks. Roofs have also been ripped off houses.

"It's a mess. Apparently a car went 30 feet in the air and smashed down," he said.

Police were urging 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.

The suspected tornado that ripped through Durham worked its way east to the top of Blue Mountain before barrelling down into the town of Craigleigth located between Thornbury and Collingwood, Ont., police said.

"It was a large funnel cloud, coming over the top of the mountain and then touching down ... shearing off the tops of homes," said provincial police Sgt. Chris Maecker.

The twister then went out across Georgian Bay and over to Gravenhurst, which was reporting significant damage as well, Maecker said.

Although about 20 homes were destroyed and a number of barns flattened, no one seems to be hurt in the Collingwood area, he added.

The east side of Gravenhurst appeared to be the worst hit with residents reporting extensive damage to houses and hydro lines snapped in half.

"It sounded like a train or a jet," Innkeeper Tyler Allair said.

Residents remained in the dark as hydro crews worked to restore power to the town.

"It's pretty chaotic and there's no power," Allair said.

Hotels on Georgian Bay also reported minor damage from the gales that shook their grounds earlier in the evening.

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

Vaughan Emergency Line: 905-832-2281