The members of Cheap Trick got off the stage safely, but witnesses said the musicians were thrown off their feet.
"Everyone is okay and we are so lucky to be alive and hope that all the fans are okay too," the band, best known for hits including "Surrender", "I Want You to Want Me" and "The Flame", said in a message posted on Facebook.
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.
Jocelyne Turner, a spokeswoman for Ottawa Fire Services, said five people were treated at the site. Three of them were sent to hospitals and one had life-threatening injuries.
The Globe and Mail newspaper of Toronto reported that a 46-year-old man was in serious condition, suffering a pierced abdomen, a fractured pelvis and a fractured leg after he was gored by a piece of the collapsing stage.
Environment Canada had a thunderstorm warning in effect for Ottawa, saying winds were expected to reach 56 mph (90 kph).
Bluesfest is one of North America's main musical events. The festival first took place in 1994 and has since grown from a one-stage, three-day event to a multi-staged, 12-day music showcase featuring some of the best international talent.