More than 100 people were forced out of their homes Monday after a four-alarm fire broke out at their St. Leonard apartment building.

The blaze started around 4 a.m. on Belanger St. in a three-storey building with 30 units.

The fire starting on the first floor and quickly spread to second and third floors, then the roof.

The first started in the eastern wing of the building. The eastern and western wings of the building were separated by a firewall, which firefighters say prevented the blaze from spreading to the other side.

Four people had to be brought to hospital to be treated for burns and smoke inhalation.

More than 100 firefighters were at the scene at the height of the blaze.

Sun Youth and the Red Cross are at the scene helping those who have been displaced.

A spokesperson for the Montreal fire department said the building was most heavily damaged on third floor and roof. The damage was enough that an excavator is being called to demolish half of the building – built in the 1960s, inspectors determined that the fire compromised the building’s structural integrity.

Residents were escorted inside to collect their personal belongings before the building's demolition started.

One resident said it was sad to return to his home and be met with charred belongings and the smell of burning.

“You realize that your life can change in an instant,” he said.

It is still unclear when residents of the undamaged units will be allowed back into their homes.