Paramedics help one of three people injured when lightning struck in a Brampton, Ont., park. (Sean Magennis)

Two young boys and the mother of one were seriously injured Wednesday afternoon when lightning — described by one witness as a "fireball" — struck a soccer field in Brampton, Ont.

The boys, aged four and six, were in critical condition in the Hospital for Sick Children in downtown Toronto, while the mother was in serious but stable condition at Brampton Civic Hospital, said hospital spokesman Todd Leach.

Peel regional police Const. Jodi Dawson told CBC News the four-year-old boy's babysitter witnessed the lightning strike but wasn't injured.

The trio were struck on a soccer field at Centennial Park, near Queen Street East and Main Street at about 2:20 p.m.

Witnesses are consoled after a lightning strike Wednesday afternoon. (Sean Magennis)

Allan Hughes, whose backyard is next to the park, said he was watching a Toronto Blue Jays game when he was startled by an "abnormally loud bang." He then heard a woman screaming for someone to call 911.

Hughes said he then ran out to try to help after the incident and discovered one of the boys lying in the field not moving.

The boys' clothes had been shredded off due to the impact of the lightning strike, Hughes said.

"[It] was just the worst, terrifying thing I've ever seen," a visibly distraught Hughes told CBC. He tried to do CPR on one of the boys.

"His little eyes were wide open, blue eyes just staring at me."

Connie Denbok, whose home also borders the soccer field, said it was partly cloudy at the time with no rain, but there was a distant rumbling of thunder.

Then Denbok said she saw "what looked like a fireball outside my window," followed immediately by a clap of thunder and screaming.

She hurried to the field and sat with the three who appeared unhurt, though she said they were understandably quite upset.

Storm a 'weak and localized event'

A thunderstorm was reported at Pearson International Airport just east of Brampton at 1:46 p.m. ET.

Although there was a forecast 40 to 60 per cent chance of thunderstorms in the Toronto area Wednesday, there were no specific watches or warnings issued for Brampton, west of Toronto, according to CBC meteorologist Nick Czernkovich.

"This particular thunderstorm didn't meet the warning criteria, but it doesn't mean it couldn't be dangerous," said Czernkovich. "Every thunderstorm can be dangerous and should be treated with caution."

The No. 1 safety tip is to get indoors, he added.

Environment Canada meteorologist Geoff Coulson said the storm was considered "weak and a very localized event" that only produced 15 to 20 flashes of lightning.

"Every thunderstorm has lightning associated with it," Coulson said. "There would be too many warnings put out, so that is why we reserve watches or warnings for only the most severe situations."

With files from The Canadian Press