The section north of the Lachine Canal, extending up to Notre Dame Street, will be reduced to three lanes until November. Traffic going south will be directed to Nazareth Street.
Crews started demolishing the northbound portion of the elevated highway this morning and will knock down the structure, one section at a time, between support joists.
As each section is demolished, reconstruction will begin there.
The portion of the expressway that belongs to the City of Montreal was constructed in 1966.
"The city decided this is an urban area and the idea of having an overhead expressway like this is not appropriate anymore," said Pierre Sainte-Marie, the head of the city's transportation infrastructure division.
"The idea is to bring it down and that is literally what is starting this morning."
The new version will feature green space and nine ground level lanes. It will have pedestrian corridors and connect to new cycling paths on William Street and Ottawa Street.
The roadway will still be elevated to pass over the canal, but it will descend to ground level at Wellington Street instead of Notre Dame.
The northbound portion is slated to be completed by November. Next summer, the city will tackle the southbound expressway.
The project is scheduled to be completed in 2017.