The main stage at the Ottawa Bluesfest collapsed 20 minutes into a performance by Cheap Trick Sunday night after a sudden thunderstorm blasted the area.
Ambulances, police and fire vehicles descended on the scene and thousands of people fled the area.
At least three people were injured, one seriously, The Canadian Press reported.
CBC reporter Stuart Mills said black clouds rolled into the area and the band left the stage moments before the structure collapsed.
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"A giant gust of wind picked up a tarp and garbage from across the field and flung it toward the audience and lifted the whole stage — where Cheap Trick had been playing — and almost like a convertible in a car, just folded it backwards in the same direction," he said.
Michael Robert, who was close to the stage when it collapsed, said the "whole thing just came down in a few seconds."
Video of the Bluesfest site posted on YouTube within minutes of the storm's passing showed a stage that had crumpled and collapsed over electronic equipment. Twisted shards of metal jutted out from the stage which stood several stories tall before it was destroyed.
Cheap Trick posted a message on Facebook saying no one in the band was injured, along with a picture of some of the destruction.
"Everyone is okay and we are so lucky to be alive and hope that all the fans are okay too," they wrote.
Storm caused damage across Ottawa
Organizers of the annual event called off the show when lightning began to flash, scrapping the festival's final act, Death Cab for Cutie.
"Due to extreme weather conditions, we have regrettably had to shut down all stages for the night," a statement issued by Bluesfest said.
An Environment Canada thunderstorm warning for the area predicted winds up to 90 kilometres per hour.
The CBC's Chris Carter said trees were brought down and power was lost in several parts of the city. He said most of the damage from the storm was located in the city's south and west.
Ottawa police said they received 300 emergency calls in a two-hour period, in addition to those from Bluesfest.
Bluesfest first took place in 1994 and has since grown from a one-stage, three-day event to a multi-staged, 12-day music showcase.