It's the first extreme cold weather alert of the year and of this winter.
Temperatures are expected to drop well below freezing on Monday night. With windchill, it will feel colder than –20C, CBC meteorologist Jay Scotland said.
"There's always the risk of frostbite so keep your yourselves and your pets indoors if you can," he said.
Doug Johnson-Hatlem, a street pastor with Sanctuary Ministries of Toronto, notes that 34 people died on Toronto streets last year.
He says there are shelters available for the city's homeless — but there's not always enough space for those who need it.
"They're camping out on chairs, they're hunched over in their wheelchair trying to sleep. And sometimes it's so full there people have to hang out right outside the building and just sleep right outside the building," he told host Matt Galloway on CBC Radio's Metro Morning.
An extreme cold weather alert triggers extra homelessness services.
Shelters relax any service restrictions, the city immediately adds 172 shelter spaces, and overnight street outreach is increased.
Anyone who sees someone he or she believes requires street-outreach assistance is asked to call 311. In an emergency, dial 911.
The alert is in effect until further notice.