The Viger tunnel is closed in both directions after inspectors from Transport Québec analyzed a concrete slab.
The tunnel is closed in the eastbound direction from the Saint-Laurent exit ramp.
In the westbound direction, drivers are being detoured at the Panet entrance ramp.
Montreal Mayor Denis Coderre said the closure was unexpected.
"It seems there has been some work around the CHUM (Centre hospitalier de l'Université de Montréal on Viger Street), and now you have concrete slab that is held only by friction. So the inspectors and all the engineers said, we have no choice. We have closed it on both sides," Coderre said.
Coderre said the City expects the tunnel to reopen sometime over the weekend.
Quebec Transport Minister Robert Poëti is also on the scene and is expected to address reporters soon.
More to come.