Firefighters said on Twitter it's a three-alarm fire that began in the basement of a three-storey building under renovation.
Montreal police say the southbound section of Papineau Avenue is closed at Ontario Street.
They advise drivers to reach the Jacques-Cartier Bridge by Ste-Catherine Street or René-Lévesque Boulevard.
A spokeswoman for the Montreal fire department said there are 90 firefighters on the scene. Shortly after 8 a.m., she said the fire was still out of control
Red Cross officials are also present because surrounding buildings had to be evacuated, she said.