One person is dead and six are injured after a tanker truck carrying flammable liquids caught fire and exploded in a rush-hour collision on Tuesday involving at least four vehicles.

More than 100 firefighters were called to the site of the accident as black smoke was sent billowing over Montreal.

Fire operations chief Danny Ciavaglia said the injured were treated on the scene by Urgences-Santé.

After it sustained explosions and fire, officials say they won't reopen the overpass until they are sure it is safe. (Simon-Marc Charron/Radio-Canada)

The fire was eventually put out but Highway 40 remained closed Tuesday evening in both directions at Lajeunesse Street as firefighters grappled with what Ciavaglia called a "hazardous materials intervention" that also involved officials from the federal and provincial environment ministries.

He said Montreal's fire service cannot yet confirm the nature of the flammable liquid contained in the tanker truck.

He said firefighters used foam to put out the fire, and "it's still hot, and we can't go walking anywhere near it until it cools down."

In this video taken by a witness in a building overlooking the highway, just moments after the accident, we see the flames spreading.

[VIDEO]

'Plan alternate routes' advises Coderre in aftermath

Montreal Mayor Denis Coderre said that safety is a priority in the aftermath of the deadly accident. He advised motorists who typically drive on Highway 40 past Lajeunesse Street to plan alternative routes.

"There will be some sacrifice," he said.

A man is seen trying to pull the driver from a truck filled with diesel before it exploded in Montreal on Tuesday. 2:52

After provincial police are done their investigation into what led to the collision, investigators from Quebec's Transport Ministry will begin their work at the scene.

Quebec Transport Minister Jacques Daoust said his team will work through the night to determine the integrity of the overpass.

"One death is enough. We don't want anything else to happen," he said.

The explosion and fire occurred on the westbound side of the highway so Daoust said he hopes the eastbound side can reopen quickly, but the extent of the damage sustained by the overpass will determine how long it will be closed for.

Sewer system contaminated

The sewer system has been contaminated, Ciavaglia said, and environment officials on the scene are trying to determine the extent of that contamination.

Ciavaglia said at least one building – the headquarters for the Caisse Desjardins and the FTQ, just 100 metres from the site of the accident – has been damaged from the intense heat of the fire.

Eyewitness Patrick McQuilken, who works in the building, said at around 4 p.m., just as he was poised to call 911, he heard explosions.

"There was a series of violent explosions. Our building is 17 storeys, and our whole building was engulfed in smoke," McQuilken told CBC's French-language service Radio-Canada.

About 100 firefighters were at the scene as the fire raged around 4 p.m. (Brigitte Hébert-Carle/Radio-Canada)

Transports Québec spokesperson Nomba Danielle said the accident happened on the westbound side of Highway 40, and that elevated structure is definitely damaged.

Ministry officials will be able to evaluate the extent of the damage once emergency crews have completed their work in the area, she said.

"The minister of transport will inspect the structure as soon as possible," Danielle told CBC News.

Witnesses say as many four cars are on fire. (Alexandre Leduc/Radio-Canada)

Earlier, witnesses said as many four cars caught fire.

The plume of smoke was visible from as far away as Chambly, 30 kilometres southeast of Montreal.

Vincent Leroux, who lives in a building overlooking the Metropolitan expressway near the scene of the accident, told Radio-Canada, that the area had not been evacuated. However, he said, residents of homes in the area poured out onto the street to get a better look at what was going on.

The service roads near the accident remain closed.

DERNIÈRE HEURE : Incendie sur l'autoroute métropolitaine à la hauteur de Lajeunesse. Explosions entendues. <a href="https://twitter.com/hashtag/rcmtl?src=hash">#rcmtl</a> <a href="https://t.co/f79gfugNGv">pic.twitter.com/f79gfugNGv</a> —@SMCharronRC

Service on the STM's Orange line is now back to normal, however, until 6 p.m. trains were prevented from stopping at Crémazie, the Metro station that is only a few hundred metres away from the accident.

Fire on Highway 40 : service resuming at Crémazie station, buses still diverted to Sauvé and Jarry métro stations <a href="https://twitter.com/hashtag/STM?src=hash">#STM</a> —@stminfo

Transports Québec said Highway 40 will remain closed for an undetermined period.

The Quebec provincial police are suggesting drivers use Jean Talon or Jarry streets as alternative routes around the scene of the accident.