The city says the alert is meant to make vulnerable homeless people aware of the dangers of staying outside too long in cold weather and to ensure they have safe and warm places to go.
Toronto saw a low of -17 C overnight while Monday's high was expected to be -8 C and temperatures are not expected to rise above zero until Thursday.
The alert is in effect until further notice.
An extreme cold weather alert is called to trigger additional homelessness services for the city, giving community agencies time to call in extra staff if needed.
During such an alert the city makes additional shelter spaces available for men and women and increases outreach services in the downtown core.
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