Water poured into the tunnel, just south of the Atwater Market, and some surrounding streets were also flooded.
A contractor working in the sector accidentally broke the pipe, said city spokesperson Marilyne Laroche Corbeil.
Police and firefighters responded to a call around 10:30 a.m. and closed off streets around the sector.
Three cars were almost entirely under water but no one was injured, Sud-Ouest borough mayor Benoit Dorais said at the scene.
"You have to avoid the area. This break is not in the tunnel, its south of it, and its flooding the tunnel with water," said city spokesperson Patrick Sabourin.
Tunnel Atwater fermé en raison dune inondation suite à une conduite perforée | Je suis sur place accompagné du directeur du @MTL_SIM : aucune vie en danger; mesures durgence du #SOMtl décrétées; et les équipes sont sur place pour rétablir la situation. #polMtl pic.twitter.com/YyYHM1kNnO
The incident did not require water to be cut off from homes, nor was the drinking water affected, Corbeil said.
Montreal Police Constable Andrée-Anne Picard said police were at the scene helping direct traffic.
Montreal police are directing traffic in the area. The Atwater Exit on Highway 20 is currently closed. There are some detours in the area:
Plante said the water would be pumped into the city’s sewer system, but she did not know how long it would take before the street could be reopened.
Picard urged motorists to avoid the area. Atwater was closed between Notre-Dame St. and Joseph St. in Verdun. The Atwater exit off Highway 15 was also closed.
The Atwater Tunnel was closed on Nov. 20, 2018 when a water main broke and flooded the area. (CTV Montreal/Renaud Boulanger)
A punctured pipe has flooded the Atwater Tunnel, stranding three vehicles in around three metres of water.
Its not known when the tunnel will be reopened, but it will likely be closed for the afternoon rush hour.
The tunnel, which runs beneath the Lachine Canal, began filling up with water late Tuesday morning. It will be closed to traffic at least until this evening.
Mayor Valérie Plante, who visited the site Tuesday afternoon, urged drivers to avoid the area if possible.
No one was injured in the flooding, but the vehicles, which belong to construction workers, were almost completely submerged.
"[The situation] is under control. We're closing the valves, so no more water will be entering the tunnel," Plante told reporters.
Southwest borough Mayor Benoit Dorais shared video of the flooding on his Twitter feed earlier on Tuesday.
City officials warned motorists the water main break could contribute to icy road conditions around the tunnel.
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