"No sewer collector network would have been able to manage the quantity of water that we saw," the mayor said, adding that damage could have been worse if the city hadn't spent millions of dollars on road work in recent years.
More than 50 millimetres of rain fell in a half-hour Tuesday evening, causing water and sewage to erupt from some city sewer covers.
The metro's orange line was shut down as water poured into stations, an expressway was flooded and homes and businesses reported torrents of water pouring into their basements.
Tremblay said city crews were sent out to repair sewer pipe covers and handle the overflow.
But damages are widespread. At Montreal's Museum of Contemporary Art, hundreds of works of rare art were damaged.
"We have one to three feet of water in our basements," said Paulette Gagnon, museum director.
"Part of the basement holds our reserves so of course some of our reserves were damaged."
Environment Canada said up to 80 mm of rain fell in some areas of the island in less than an hour. Streets became rivers, metro tunnels were flooded and the Ville Marie tunnel was shut.
Jacques-Alain Lavallée, spokesman for the City of Montreal, said they are still evaluating the property damage.
He said sewers overflowed and the flooding was widespread. Four boroughs were hit the hardest: Ville-Marie, LaSalle, the Southwest and Hochelaga-Maisonneuve.
Lavallée said the problem wasn’t the city’s aging infrastructure — it was the sheer amount of rain.
Downtown universities and public institutions were also hit hard. Part of the lower level of the Grande Bibliothèque flooded.
Students writing an exam at Dawson College were ushered out of the building as the water started to rise.
Donna Varrica, spokeswoman for the college, said placement exams for incoming students scheduled for late this afternoon have been cancelled and will be rescheduled.
The gym where the exams were to take place needs to dry out, she said.
Concordia University’s library remains closed due to water damage and some classes had to be moved. The University of Quebec in Montreal also closed several pavilions and parking lots.
Several schools on Montreal’s South Shore were also forced to close because of water issues.
In many parts of the island, hundreds of people will continue to clean up flooded basements.
Michael Murray said his son called him during the storm and said water was gushing up from the toilet and shower drain.
This morning he had windows open and fans going at his home on Percival Avenue to try and dry out some of the water.
However, he said much of his finished basement will need to be ripped out and redone.
The old sewer system in Montreal West just couldn’t keep up with the deluge of rain, he said.
"Probably everyone’s sump pumps were working, flowing into the sewers and that just overflowed the whole system so it came back," he said.
Other residents said even people with specially installed back-up systems saw those systems fail and ended up with flooded basements.
Browse the map and click on rainstorm icons to read about flooding problems around town:
View Monteal flooding in a larger map