"All residents and es must conserve water. Do not shower or bath. Do not wash dishes, or run appliances that use water," the alert said.
Henry said the cause of the "large and complicated" break is still unknown.
The city also invoked a mandatory outdoor watering ban in order to conserve water for emergency firefighting needs and instituted a city-wide fire ban.
"Every drop of water will count," said Henry. "This is not the time for anything other than critical uses."
However, Calgary Mayor Jyoti Gondek said it was important to clarify the difference between the mandatory restrictions in place for outdoor use and the city's request for Calgarians to curtail water use within the home.
"We are not saying that you can't give your kids a bath," said Gondek.
Gondek also said that the provincial government is being kept apprised and is working with the city to navigate the ongoing situation.
The break caused streets to flood in the Bowness and Montgomery areas and forced the closure of several roads and inter, including 16th Avenue in both directions.
The Trans-Canada Highway remained closed from Sarcee Trail to Home Road as of 8 a.m. Thursday morning, and Calgarians are being asked to avoid the area during their morning commute.
"This is a significant break on a critical water main — called a feeder main — that moves water across the city," the city said in a release earlier on Thursday, as the city issued a boil water advisory for the Bowness area at the direction of Alberta Health Services.
Nancy Mackay, director of Water Services with the City of Calgary, said the boil water advisory will be in effect for Bowness until further notice, but the water in all other communities is safe to drink.
The break also affected water pressure in several other areas, including the communities of Parkdale, West Hillhurst, Hillhurst and Scarboro.
"The city has enacted its Municipal Emergency Plan and currently, we do not have an estimate for how long the boil water advisory or the water restrictions will be necessary," the city said in a release Thursday.
The boil water advisory requires all residents and es in Bowness to bring water to a rolling boil for at least one minute before any consumption, including:
The city could not give an estimate as to when it would be repaired.
The Calgary Board of Education says seven of its schools have been affected. Water has been shut off at:
Portable toilets and bottled water is being made available at these sites, the board said.
The water main break in Calgary has also prompted the City of Airdrie to ban all outdoor water use and to strongly encourage indoor water conservation.
With files from Colleen Underwood and Omar Sherif
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