An explosion in a rooming house on Sunday caused a major fire and injured several people.

"Two males in their 50s are suffering from major burns to 70 to 80 per cent of their bodies; first- and second-degree burns," said Urgences Sante spokesperson Bob Lamle.

The two who are seriously injured do not have burns characteristic of an explosion and were more likely injured in the fire, said Lamle.

By Monday morning one man had recovered slightly, but the other man remained in critical condition.

A total of four others were injured.

“There were three transported to hospital; two with major burn injuries, one with minor injuries. Three other people were evaluated and treated at the scene, including one firefighter,” said Lamle. The firefighter scratched himself badly.

The fire took place in a 53-unit rooming house, though it's unclear how many people were in the building at the time. Building manager Shailesh Patel said 40 of the 53 rooms were rented, but did not know how many tenants were inside.

The building does not have a sprinkler system.

The Montreal fire department was alerted to a building collapse and violent fire at 1460 Belanger St. between Garnier and Fabre Sts. at 11:53 a.m.

"When our first unit arrived on the scene, a panic was going on and people were scrambling out of the building," said Montreal fire department chief of operations Luc Galarneau.

The fire department assisted in evacuating the building.

According to the first details of the investigation, there was an explosion in a hallway near a supporting wall on the ground floor at the centre of the building, said Galarneau. There were initial concerns of a possible building collapse for that reason, but the building is secure, he said.

"That has been resolved. Firefighters from a particular special team stabilized the building," said Galarneau, adding that the team came in to erect temporary walls to help the team proceed.

As many as 100 firefighters were deployed to the scene.

Montreal police cordoned off a large area to allow emergency crews to conduct their operations safely, only blocks from where the Montreal Marathon was taking place.

Police say there is no initial evidence the fire is criminal in nature. An investigation is underway by the police arson squad to determine the cause of the blast.

The Red Cross is providing assistance to those who have been forced out.

With files from La Presse Canadienne.