The organizer of the Ottawa Bluesfest says the storm that collapsed the main stage of the festival on Sunday was a "freak situation," adding that officials made the right call to cancel the event moments before it happened.

Cisco Ottawa Bluesfest executive director Mark Monahan said the storm "came in very quickly and came out very quickly" and said festival staff did a good job handling what could have been a deadly situation.

The band Cheap Trick had been playing for 20 minutes when the storm clouds rolled in. The band hastily left the stage and, moments later, the structure collapsed.

Ottawa paramedics said they responded to multiple injuries after the stage collapsed around 7:20 p.m., including a 49-year-old man working at the show who suffered abdomen, pelvis and upper leg injuries. Another man had possible spinal injuries.

Monahan said all three people have since been released from hospital.

The site is now in the hands of the Ministry of Labour. Ministry spokesman Matt Blajer said Bluesfest organizers have been asked to hand over any videos they might have of the collapse and any engineering documentation such as technical drawings of the site.

The westbound lanes of the Ottawa River Parkway were closed between Booth Street and Bayview Street on Monday morning as investigators probed the stage collapse.

Monahan said the stage was a rental and inspected daily by Bluesfest staff, but said he did not have any further information on the technical aspects of its structure.

He said the incident has never happened in Bluesfest's 18-year history and called the storm "a very unusual situation...just a freak situation."

CBC reporter Stuart Mills said black clouds rolled into the area and the band left the stage moments before the structure collapsed.

"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," Mills said.

Concertgoer Leanne Wilson said the stage slowly heaved backward and caved in.

"In less than 10 seconds it was gone," she said. "[Cheap Trick] were playing right until it fell. And then instantly everybody was just running and screaming."

"I was stunned, I'd never seen anything like that before at a concert."

Cheap Trick manager Dave Frey told CBC News that the wind hit the stage "so hard that it just kind of creaked and groaned and then you heard rivets popping."

Environment Canada said a downburst was likely the trigger for the collapse of the main stage.

A downburst is a particularly strong downdraft of air from a thunderstorm that spreads out in all directions when it hits the ground and can potentially cause as much damage as a tornado. Thunderstorms with wind gusts up to 96 km/h began around 7:30 p.m. ET and continued on and off through the evening.

Ottawa Mayor Jim Watson was on the main stage 20 minutes before the storm and then moved with Bluesfest organizers to a viewing tower set up across the stage but left the tower as the winds rose.

He said an investigation is underway, but that he thinks the sudden storm would have been difficult to anticipate.

"Some of these circumstances you can't plan for ... 90 [km/h] winds generally don't happen in our city and the main stage was the primary area affected because it was so large," said Watson.

"There was another stage not quite as big next to it that seemed to have no damage and it withstood 90 km/h winds."

He praised emergency workers, organizers and the fans for how they handled the situation.

"You could imagine the thousands of people trying to leave the stage area. We're lucky there was no trampling. People acted thoughtfully and respectfully to their fellow fans and we are very fortunate it was not worse than it was," said Watson.

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.

Organizers of the annual event called off the show when lightning began to flash, scrapping the festival's final act, Death Cab for Cutie.

Trees were brought down and power lost in several parts of the city, with most of the damage from the storm was located in the city's south and west.

Environment Canada said the storm caused extensive damage across west Quebec, including uprooted trees, upturned boats and mobile homes. A tornado was also reported north of Val D'Or, Que., Environment Canada said.

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.

With files from The Canadian Press