A spokesperson for Montreal's fire department says the leak happened at 2100 Mont-Royal Ave. while maintenance work was being done on the building's heating system.
The building, which houses a National Bank on the ground floor and two apartments above, had to be evacuated.
Firefighters and inspectors from Gaz Metro are still on the scene.
The intersection of Mont-Royal and de Lorimier avenues is closed to traffic.
More to come.