Two people were killed and 29 others were injured in a colossal pileup on Montreal's South Shore on Wednesday.

Provincial police said the wreckage spanned about one kilometre, bringing 140 vehicles to a screeching halt on Highway 15 as blinding snow blew from the shores of the St. Lawrence River onto Highway 15.

Roughly half of the motorists involved managed to avoid disaster and were able to drive away several hours later when the road was cleared.

A school bus was near the front of the pileup, pressed against a damaged tanker truck, but police said nobody on board was injured.

The two who died were in the same vehicle, police said. Police have yet to release details about the victims.

"We know that these people were involved in a collision with a tank truck, which made the rescue operation more difficult for first responders," said Stéphane Tremblay, spokesperson for the Sûreté du Québec.

The estimated number of injuries has varied throughout the day, with the final tally hovering at 29.

The pileup began at kilometre 46 in the southbound lane at around 12:30 p.m. ET in the suburban city of La Prairie.

A 10-kilometre stretch of the highway was immediately closed in both directions. At first, transport officials thought it might stay closed until the next morning, but have since said it will open by the end of the night.

Danielle McCann, the province's health minister, said three hospitals in the area initiated a Code Orange — a hospital-wide warning that alerts staff that multiple casualties are on the way.

"It was a very prompt operation," she said, commending rescue workers.

Firefighters had to extricate victims from at least nine vehicles. Police say about 150 people in all were tended to at the scene. Many, including those with minor injuries, were taken to a local community centre by chartered buses where a makeshift triage centre was setup.

Blinded by sudden whiteout

Though the incident is still under investigation, authorities say it was likely a sudden whiteout caused by blowing snow that led to the chain reaction of collisions.

"I spoke to a few people who couldn't see anything, and then they saw trucks up close. These people were immediately pulled aside and looked after," said Sébastien Lavoie, a division chief with the La Prairie fire department.

WATCH | More than 100 vehicles involved in pileup on Montreal's South Shore:

Emergency responders work to get people out of their cars after a multi-vehicle pileup in La Prairie, Que. 0:29

Snow-clearing vehicles had passed through the area twice in the hour before the collisions took place, said Quebec's Transport Minister François Bonnardel at an afternoon news conference.

He noted a chain-link fence that runs along the highway which is there to help block the blowing snow that gets caught up in gusts of wind from the St. Lawrence River.

The area is not considered prone to collisions as there have been no major accidents there for at least two decades, Bonnardel said.

Mayor calls for improved safety

La Prairie Mayor Donat Serres disagreed with the minister later in the day.

He said the highway was reconfigured in the early 2000s. Its height was raised, he said, creating a "springboard effect" that propels snow up to the height of windshields as strong winds blow off the vast, open section of river.

"I think we need an intervention plan and that we find solutions," he said, suggesting windbreakers be installed along the highway.

Firefighters had to extricate victims from at least nine vehicles. Police say about 150 people in all were tended to at the scene. (Simon-Marc Charron/Radio-Canada)

Stéphane Gagné, a driver who was caught up in the pileup, told Radio-Canada the visibility was intermittent on the highway and the pavement was extremely slippery.