A stage at Toronto's Downsview Park has collapsed in advance of this evening's scheduled Radiohead concert, crushing a man in his 30s to death and injuring three others.

Calls came in to emergency crews at about 4 p.m. ET, and at least one ambulance was already on site. Police say a 30-something man was declared dead at the scene, and a 45-year-old man was transported to Sunnybrook Hospital with a serious but non-life threatening head injury. Two others had minor injuries and were released without treatment.

The man's name is not being released until police notify his family.

The 7:30 p.m. concert is cancelled, and Radiohead's official Twitter feed advised fans not to make their way to the venue.

Gates had been scheduled to open at 5 p.m.

Alexandra Mihan was setting up in a beer tent parallel to the stage when she heard a sound like fireworks.

"Then everyone started screaming and gasping," she told CBC News. "We turned around, and the entire top of the stage has just collapsed. All of the metalwork and all of the screens had just kind of folded over on top of each other."

Food vendor Jeff Cole said between 10 and 15 crew members were on the large stage, which included screens and extensive lighting.

"It was like watching in slow motion," he said. "The top just came straight down on the right side. The metal was all mangled. I'm staring at it right now, and it's not a pretty sight."

Firefighters went into the structure to extricate the man who died. It wasn't clear whether the man was still alive when they found him.

The area was quickly cleared of witnesses by emergency crews.

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Concert-goers at Toronto's Downsview subway station head home from Saturday's scheduled Radiohead show, which was cancelled due to a fatal accident. (CBC)

"It's very, very fortunate that the gates weren't open, because it would have been pandemonium," Cole said.

Police say the park wasn't full but there was a considerable crowd of people already waiting for the show. The park is a frequent site of concerts, and 40,000 people were expected for tonight's sold-out show by the hugely popular English band. The opener was to be Canadian act Caribou.

The weather in Toronto at the time of the accident was calm, with temperatures in the high 20s and the forecast calling for light winds.

Mihan said the weather was "beautiful."

"It couldn't have been the weather that attributed to the collapse."

Toronto police Const. Tony Vella said the force will work closely with the Ontario Ministry of Labour to determine the cause of the disaster. He urged anyone who witnessed the collapse to come forward.

Vella said he didn't know yet who the temporary stage belonged to.

Several stage accidents

The stage collapse is one of several high-profile accidents at concert sites throughout North America in recent years — though the others happened during inclement weather and with fans nearby.

In August 2009, the collapse of a stage at Alberta's Big Valley Jamboree in Camrose, Alta., killed a 35-year-old woman when a storm blew in.

In July 2011, a stage at Ottawa Bluesfest fell in a storm just moments after the band Cheap Trick had left the stage. No one was killed, but several people were treated for injuries.

On Aug. 13, 2011, a wind gust toppled the main stage at the Indiana State Fair, killing five people and injuring 45 other as they were waiting for the band Sugarland to perform. A study determined the stage was poorly designed.

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Riggers and technicians set up the stage earlier this week for the doomed Radiohead concert. (Downsview Park/Facebook)

With files from The Canadian Press