A violent storm of wind and hail hit an outdoor rock music festival on Thursday in northern Belgium, leaving at least two people dead and 40 others injured, 11 seriously, local officials said.

Two stages collapsed, one falling on the concert-goers. Some giant screens also fell down and trees were uprooted by the fierce storm, the Belga news agency reported.

Tens of thousands of people had flocked to the annual outdoor concert, the Pukkepop, held in Kiewit near the town of Hasselt.

"The storm at the Pukkelpop left two dead and 40 injured," said Hasselt Mayor Hilde Claes, according to the news agency.

Firefighters had earlier reported one death and seven seriously injured.

"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, drew between 60,000-65,000 people on its opening day Thursday. The organisers said they will decide whether the festival will continue as planned through Saturday.

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.

Just 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 Sugarland country music concert, killing five people and injuring another 45 concert-goers.