An overpass collapsed over a highway north of Montreal crushing cars below and likely killing the occupants while injuring at least five people, according to authorities.

Quebec provincial police did not confirm any deaths but spokeswoman Chantal Mackels said police believed it unlikely that any of the occupants of the two cars below could have survived the crushing weight of the concrete.

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Drivers on Highway 19 slammed on their brakes and watched in horror as the overpass slowly collapsed, sending cars plunging and crushing at least two vehicles.

"We watched the overpass slowly come down and two cars fell from it," witness Golda Simon told TVA network.

Ambulances quickly arrived on the scene and transported five people, three in critical condition, including two suffering from head injuries. Emergency workers were yet to reach people stuck under the wreckage, said Andre Champagne of ambulance service Urgence-Sante.

Emergency workers were trying to secure the site before attending to people caught in vehicles under the wreckage, he told AP.

Mackels said three lanes of the overpass plus a pedestrian sidewalk collapsed onto Highway 19. She said four vehicles were on the overpass at the time and that the concrete collapsed on at least two vehicles underneath.

The incident occurred before 1 p.m. (1700 GMT) One witness told TVA he noticed that the road sunk by 1 to 2 inches (2.5-5 centimeters) when the traveled over the overpass minutes earlier and called emergency dispatchers at 12:30 p.m. (1630 GMT).

Manon Joly had just dropped her daughter off at dance school and was in her car when the drama unfolded. She said she was about to drive onto Highway 19, when "I slowed down and then all of a sudden everything collapsed like a house of cards.

"There were three or four cars that plunged into the hole right in front of me," she said.

Aerial helicopter shots showed two passenger cars and a van on their backs or on the sides over the wreckage of the collapsed overpass, blocking traffic for miles. Authorities closed the bridge further down connecting Laval with the island on Montreal.

The incident was similar to the 2000 collapse of an overpass, also on the island of Laval north of Montreal, killing one and injuring two.