Cars literally fell out of the air. Apartment windows rattled. Witnesses compared the sonic boom to a bomb.

An entire storey of a Montreal parking garage collapsed on Wednesday, with a giant slab of concrete dropping to the floor below, crushing vehicles and killing at least one man.

Authorities described the collapsed floor as a giant, falling pancake - a plummeting concrete chunk 30 metres long and 30 metres wide in size and untold tonnes in weight.

The split-second tumble triggered immediate chaos: 500 people evacuated from an apartment highrise; toddlers whisked from a daycare centre; and people screaming and crying that they'd narrowly escaped death.

Rescue crews tried frantically to reach a man trapped in a car. They used jackhammers to break through the concrete and circular saws to slice through his vehicle door.

The man sat slumped in the driver's seat and did not respond when they shouted out to him. He was declared dead after crews made it into the car.

The man in his 30s had just arrived for work at a dental-supplies courier company that used the two-storey underground garage to park its vehicle fleet.

One of the stories collapsed at around 8:45 a.m. The concrete slab dropped, causing a loud boom and then shock among motorists who'd just parked their cars and barely averted disaster.

"It caved in - sort of like a pancake," said fire department spokesman Aime Charette.

"It fell as if you'd just cut out a piece."

The collapse occurred in the basement of a 17-storey apartment building named Joie de vivre.

The highrise in suburban west-end Montreal rents out apartments at between $500 and $760 a month - and parking spots starting at $45.

One resident on the seventh floor said his windows began to shake when he heard an explosion that sounded like a bomb.

From the entrance to the multi-storey garage, a grey car could be seen flipped at a 45-degree angle and perched atop another vehicle after tumbling from the top storey.

The building owner - real-estate giant Cap Reit - initially declined to comment but said it might issue a statement.

The Toronto-based firm holds more than 27,400 homes and touts itself as providing safe, secure and comfortable accommodation.

Authorities took the precaution of evacuating the apartment building and its hundreds of residents.

Montreal firefighters got the call at 8:47 a.m. and sent more than two dozen vehicles and 80 firefighters to the scene.

One fire department official explained why firefighters were not immediately able to approach the accident site.

"We need to make sure it's not dangerous," the official said on condition of anonymity.

"We just want to make sure the place is safe to go and get the other person. We're working on it now."

Mayor Gerald Tremblay said it would be at least 24 hours before residents could be told when they would be able to return home.