Torontonians are being cautioned to stay indoors and keep hydrated Sunday as the city experiences extremely high temperatures.

Dr. David McKeown, the city's medical officer of health, has sounded the alert, which is issued when high temperatures increase the chance of weather-related deaths.

Temperatures are expected to exceed 30 C.

"We do remind people that they should avoid being outdoors in the hottest part of the day," said Red Cross spokeswoman Tanya Elliott. "And to work and exercise in brief periods and take frequent breaks. It's important to drink cool fluids."

Elliott said people without air conditioning who need a break from the heat should find reprieve in shopping malls, community centres, libraries and other cooled public spaces when an alert is declared.

Medical officials encourage families and neighbours to contact those who live alone — especially seniors, who are at a greater risk of suffering from heat-related illness.

Elliott said the Red Cross will distribute bottled water to those who live in isolated conditions or outside.