Toronto paramedics say one person is dead and another is seriously hurt after an outdoor stage collapsed while it was being assembled for a Radiohead concert.

Emergency services official David Viljakainen said a man who was trapped under the rubble on Saturday was pronounced dead at the scene.

Officials said he was in his mid-thirties. They say three other people were injured and are being assessed.

Tony Bellavance, a fire service platoon chief, said crews had assisted in extracting the victim.

"It was a crushing injury that killed the man", he said.

Police spokesman Tony Vella said crew were setting up the stage, specially built for the British rockers, when it collapsed.

Dusty Lalas, an employee with Toronto Radio Station "The Edge", a concert sponsor, said the structure suddenly caved in.

"There was a loud crash and it sounded like sheet metal and lightning and we just saw the stage collapse," Lalas said.

The three-time Grammy-winning band tweeted that "due to unforeseen circumstances tonight's concert at Downsview Park tonight has been cancelled. Fans are advised not to make their way to the venue".

Radiohead's website had listed the concert as being sold out, with sales of 40,000 tickets.