The St. Patrick's Day parade came to a halt Sunday when a 20-year-old man became fatally trapped under a float.

Police have not confirmed the man's indentity.

The incident occurred on Ste Catherine St. near the corner of Metcalfe St. at 2:15 p.m., where he became trapped under the Marianopolis College float. The young man was a spectator and not part of the parade. He and a friend decided to jump on the truck between the platform and the cab a few streets west of Metcalfe, reported CTV's Maya Johnson.

When the truck reached the intersection at Metcalfe, the pair jumped off, but the young man became pinned, said Johnson.

"I saw a person fall off, and I saw (the truck) go over, go up and then down," said Elizabeth Steinberg, who witnessed the scene.

"I saw a hand - that was it. I was yelling 'Stop the truck! Stop the truck!'... I'm shaking right now. I'm traumatized."

Montreal police Const. Anie Lemieux said conflicting eyewitness accounts had made it difficult to determine exactly how the man became pinned.

"The information we have so far is not all that clear... The information we have is that at one point he was on the float. It's very early in the investigation to figure out how all this happened. There are a whole bunch of possibilities... Collision investigators have quite a lot of work to do now," said Lemieux.

Alcohol was a factor in the young man's death, said police.

The area was cordoned off and people were evacuated from the area, said Gerald Showers from the United Irish Societies.

The perimeter was lifted some four hours later.

The incident cast a pall over the  186th annual parade, where thousands of Montrealers had been celebrating the usually festive day.

The parade began at noon and ran east along Ste. Catherine St. from Fort St. to Phillips Square.

A similar incident occurred at the same parade the in 2002, when a university student was run over by a truck and his legs were crushed under the wheels. That parade was halted three quarters of the way through due to the incident.