A violent storm that lashed an outdoor rock music festival in northern Belgium killed at least five people after uprooting trees and collapsing two stages, local officials said.

Tens of thousands of people were attending the outdoor Pukkepop festival when the storm broke Thursday, toppling one stage on concert-goers at the annual event held in Kiewit near the town of Hasselt.

"The storm hit Thursday afternoon on the Pukkepop festival, held near the town of Hasselt, and killed at least five people," Hasselt mayor Hilde Claes told local radio amid reports that more than 40 people were also injured, several seriously.

Up to 65,000 people, mainly youths, were thought to have been present at the festival when the storm hit, blowing down giant screens and uprooting trees, the Belga news agency reported.

"The sky suddenly turned pitch black and we took shelter waiting for the rain. Then suddenly, there was a downpour. The wind blew violently," said festival-goer Catherine Blaise, according to Le Soir newspaper.

"There were hailstones bigger than a centimeter falling... Trees toppled over. It was unbelievable, the end of the world," she said.

Many of the concert-goers were under a big tent, sheltered from the rains and listening to the group Smith Westerns, when it collapsed, according to the RTBF public television website.

Ambulances rushed to the scene as the concert came to a halt.

A crisis centre has been set up in a gymnasium in Kiewit to help those who have been lightly injured or in shock.

The festival, set to run through Saturday, was immediately suspended. Organisers said later that it had been cancelled.

"We simply could not carry one," said organiser Chokri Mahassine.

Belgian Prime Minister Yves Leterme issued a statement offering his condolences to the families of the victims.

Last year at the festival's 25th season, two tragedies occurred: a sound engineer died of a heart attack, and a rock singer, Charles Haddon, committed suicide by jumping from the top of a pylon after his group performed.

Last week in the United States, a similar tragedy occurred at the Indiana State Fair. A strong gust of wind brought down the stage rigging for a concert by country music stars Sugarland, killing five people and injuring 45 others.