The incident resulted in brownish water gushing from fire hydrants and an unpleasant aroma wafting, at least temporarily, over parts of the city.
The boil-water advisory applied to most of the island and even some neighbouring areas. Local officials described it as unprecedented in the recent history of the city.
"To my knowledge, it's the first time that we have in Montreal a boil-water advisory for so many people," said Valerie De Gagne, a spokesperson for the city.
She said the advisory applied to 1.3 million people, and would remain in place until at least Thursday morning. She said tests would be conducted in the meantime to determine whether the water was, in fact, contaminated.
"Once we get the results," she said, "we will be able to maintain the advisory or take it down."
Residents were advised to boil their water for at least a minute, or to use bottled water. They were also asked to avoid brushing their teeth with tap water.
However, tap water could still be used to wash dishes, take a shower or wash clothes.
The city said the advisory was prompted by abnormally low water levels at a treatment plant in the city's west end.
"The water (at the plant) dropped to a very low level. The sediments that were at the bottom ended up in the aqueducts," De Gagne said.
The affected Atwater station is the second-largest plant of its kind in Canada. It has been undergoing repair work. It remained unclear why the water dropped to such a low level, and whether the incident involved human error.
Equally unclear, Wednesday, was the water in the city. A number of residents reported seeing a brownish liquid gushing from their taps.
The fountain around city hall, for instance, looked like it was filled with mud.
While some Montreal boroughs initially started purging their fire hydrants, they were asked to stop to ensure adequate water supplies for the city, De Gagne said.
The gushing hydrants created bizarre scenes in the city, as rapids temporarily rushed down sloping streets.
It was reminiscent of an incident last winter, where a problem during renovations at a reservoir on Mount Royal caused far bigger flooding with frigid water in the city's downtown.
It was also the second major infrastructure problem in Montreal in less than 24 hours. The previous evening, the city's metro system had suffered a complete shutdown at the height of rush hour because of computer issues.
There was yet another glitch Wednesday: the city website, which carried the water advisory, appeared to be down during the morning.
Social media conversations were awash with locals fretting about the state of their scandal-plagued, service-disrupted city.
The frustration only mounted when there was another subway interruption Wednesday evening — this time when someone apparently stepped onto the tracks.
"I hope Montreal's still under warranty," one man tweeted in French, "because it's clearly broken."
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