The City of Montreal says the problem was caused by workers doing construction on Rachel St. St-Denis and St-Laurent boulevards.
"It was just a little human error. Workers closed one valve too many," said Chantal Rouleau, executive committee member responsible for water and water infrastructure.
"A valve that was not supposed to be turned off was by mistake," Montreal Mayor Denis Coderre tweeted.
Service is expected to be restored in the coming minutes.
More to come.