The St-Jacques overpass will be closed between Girouard Avenue and Décarie Boulevard in both directions until 2017, affecting about 21,000 motorists who use the route every day.
The overpass closed on Friday at midnight for a two-year- period so it can be dismantled and rebuilt as part of the reconstruction of the Turcot interchange.
The closures will affect drivers using the St. Jacques exit of the Ville-Marie expressway, as well as those travelling east along St-Jacques.
Drivers will be forced to detour onto Décarie Boulevard, Upper-Lachine Road and Girouard Avenue.
Pedestrians will still be able to use all of St. Jacques until the end of this year.
The work had been originally scheduled to begin last July.