Crew members ran for their lives when overhanging metalwork crashed onto a stage in a Toronto park Saturday afternoon, pinning and killing one man, authorities said.

The collapse happened around 4 p.m., one hour before spectators were set to begin streaming in for a concert by the alternative rock group Radiohead.

Several people were on the stage at the time, preparing for the show, when scaffolding-like material towering about 50 feet above collapsed.

"Unfortunately, four people were hurt," Toronto police Constable Tony Vella said. "The remainder of the people, when they heard the stage coming (down), ran from the area."

Firefighters arrived to find one man "trapped under the structure," said Toronto fire Platoon Chief Tony Bellavance. They helped to extricate the man, then moved away from what was then still considered an "unstable structure," Bellavance added.

Paramedics, who happened to be at the scene in preparation for the concert, "immediately rendered aid," according to on-site Toronto Emergency Medical Services commander Peter Rotolo. The victim -- who has not been identified, amid efforts to contact his next of kin -- was pronounced dead at the scene. Police said he was in his 30s.

Another man who suffered serious injuries due to the collapse was transported to Toronto's Sunnybrook Hospital, Ian McClelland of the city's EMS department said. The 45-year-old man suffered a head injury that isn't considered life-threatening, according to Toronto police.

Two other men with minor injuries were assessed and released, McClelland said.

Aerial footage afterward showed that some metal framing -- some of it covered in a blue material -- crumpled on the stage, which was in front of a large grassy area. Some of the scaffolding-like material remained standing, reaching about 50 feet in the sky.

The stage was being set up especially for the Radiohead concert, Vella said. At the time it came down, the weather was good with no storm rolling through or significant winds, added fellow police Constable Harrison Ford.

"The big question is how" this happened, Vella said. "And that's something we will be working closely with the Ministry of Labor to determine (the cause) exactly, to prevent any future cases."

Reports: Indiana State Fair stage where 7 died was inadequate

The gates were scheduled to open at 5 p.m., allowing fans onto the concert grounds for the sold-out concert, according to CNN affiliate CBC.

The show was canceled due to what the production company Live Nation Entertainment described as "unforeseen circumstances."

"Radiohead concert canceled. There has been an incident, more details forthcoming," said a post on Downsview Park's official Facebook page.

The band was to be joined by the Canadian musician known as Caribou.

The Toronto incident comes 10 months after metal scaffolding supporting stage lights fell onto a crowd of fans and workers as a storm swept through at the Indiana State Fair right before the band Sugarland was to perform.