Canada Day crowds in the capital will face the challenge of keeping hydrated in temperatures that are forecast to peak at 36 C, with a humidex of 47 and a UV index of 10, or "very high".
A government heat warning is in place for the region, but meteorologists are forecasting a drop in temperatures at the end of the week. Daytime high temperatures near 30 degrees are forecast, with humidex values in the mid to high thirties.
Montreal health officials said there have been 12 heat-related deaths since the weekend, while authorities in the Eastern Townships region east of Montreal are attributing another five deaths to the weather.
An emergency coordination centre has been activated to supervise those measures and Montreal Mayor Valerie Plante has called on Montrealers to show solidarity with at-risk people. The person had complained of the heat and breathing difficulties and died after paramedics had arrived.
She says there has also been a rise in the number of calls to ambulance services in recent days and that the city wants to avoid what happened in 2010, when intense heat claimed 106 lives.
So far the heat has only broken one record in Toronto - on Friday, Pearson airport hit a high of 35.4 degrees.
He says the new system is more accurate.
In Toronto and Ottawa, temperatures have remained well above 30 C since Friday.
Environment Canada warns this extreme heat event will continue though for most of the week, calling this streak the most significant heat event in the past few years.
Never leave people or pets inside a parked vehicle. The risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.