City of Montreal spokesman Philippe Sabourin said he expects the cleanup will take all night as crews clear debris and re-salt streets and sidewalks.
Earlier Monday afternoon, water gushed onto the streets. A large area was affected, from Sherbrooke Street to René-Lévesque Boulevard, and between Peel Street and Union Avenue.
The streets running east to west have been re-opened to traffic, but other side streets remained closed.
Earlier in the day, as water flooded onto the streets, Mayor Michael Applebaum held a press conference and urged residents to be careful.
"We will continue to work without stop," Applebaum said.
The mayor said the cause of the flooding appeared to be a break in a 48-inch water main, located in a construction zone on Pine Avenue near Peel Street.
Officials are still trying to figure out what caused the break.
McGill expects extensive damage
Much of the flooding happened close to McGill University's downtown campus.
A note sent to employees at McGill warned of "severe flooding" on the campus and inside several buildings.
"We are trying to assess damage as best we can, but it will be extensive," the note stated, warning employees to get to safety.
The note also asked employees on main and basement floors to move computers and documents as high as possible.
All evening classes at McGill were cancelled.
The Montreal transit authority reported the 15, 24, 61, 125, 168, 420, and 435e bus lines were affected by the flooding.