An 11-year-old boy died when a string of severe thunderstorms -- and two confirmed tornadoes -- ripped through parts of southern Ontario, tearing roofs off houses, overturning cars and prompting at least two municipalities to declare states of emergency.

The boy was attending a day camp when a F2 tornado hit a conservation area in the town of Durham, south of Owen Sound, Ont., on Thursday afternoon. F2 tornadoes pack winds in the 180 to 240 kilometres-per-hour range.

Ontario has been struck by at least 10 twisters this summer, leading to four deaths. Environment Canada is warning more could be on the way.

"We could easily see that number climbing," meteorologist Geoff Coulson told The Canadian Press, referring to the season's number of tornadoes.

On Friday, Grey County EMS manager Mike Muir said "a number of injuries occurred" in the conservation area where the boy died.

"Those injuries were as a direct result of the storm and the storm damage, with buildings collapsing as well as debris flying around the area," Muir said at a news conference in Durham on Friday morning.

Police say the boy was killed by debris.

West Grey Police Chief Rene Berger said emergency crews performed CPR on the boy just before 4 p.m. Thursday, but he could not be saved. He was pronounced dead at the scene.

Berger said the family does not want the boy's name released.

Also in Durham, the tornado destroyed a printing press facility and tore the roof off of a fitness centre, while a police officer was working out inside.

The town is under a state of emergency on Friday as a result of the damage.

Deputy mayor Dan Sullivan said residents are being asked to stay away from damaged areas so that emergency crews can continue cleaning things up. "We have people who know what to do out there -- just give us room to do it," he said.

Hospital officials said at least six people were injured in the storm.

Path of destruction

The system that hit Durham moved west to east across southern Ontario on Thursday, moving with such a destructive pace that Environment Canada at one point advised people to take shelter in basements or other secure areas.

Environment Canada meteorologist Geoff Coulson said that the storm system started in the Sarnia and Windsor, Ont., areas and simply got worse as it moved eastward across southern Ontario.

The presence of a very warm and humid air mass sitting over the southwestern and south-central parts of the province was the key ingredient in the recipe for yesterday's funnel clouds, Coulson said during an Friday morning interview with CTV's Canada AM in Toronto.

The suspected tornado that hit Durham moved east to the top of Blue Mountain before hitting the town of Craigleith, located between Thornbury and Collingwood, Ont.

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

The twister then moved across Georgian Bay and across to Gravenhurst, causing more damage along the way, Maecker said.

'Like a scene out of a movie'

A state of emergency is also in effect in Vaughan, Ont., just north of Toronto, where the storm tore the roofs off houses, uprooted trees and overturned cars. At least one tornado is believed to have touched down there.

Vaughan Mayor Linda Jackson said Thursday's events caused "millions and millions of dollars worth of damage" to the area.

"I'm presently in the Woodbridge area this morning and just coming here, it looks like a scene out of a movie," she said in a phone interview. "It's absolutely unbelievable the destruction that has happened in my community."

At a news conference in Vaughan later Friday, Jackson said hundreds of local houses had significant damage during the storm.

She said that 600 houses had been severely damaged and at least 44 homes would need to destroyed.

Building inspectors were surveying four damaged areas of the city.

Jackson said it was a "miracle" no one was killed in her city.

Rob Damiani's parents had the roof torn off their house in Vaughan.

"My parents were inside the house, they were down in the basement, thank God," he told CTV's Canada AM from Vaughan on Friday morning. "When they got out, everything was gone in just a split second."

He said they had been living there for 30 years.

Based on the TV storm footage available on Thursday night, Coulson said the funnel cloud seen in the Vaughan area and in nearby Woodbridge, Ont., was "obviously" a tornado.

Others still need to be confirmed, he said.

With files from The Canadian Press