About a third of France is under the national weather service's heat red alert on Sunday and temperatures are expected to reach 41C on Sunday in some areas, in a country where air-conditioning isn't widespread.
The forecast for Monday is even hotter, with forecasters saying temperatures of 39-to-40C would stretch from the southwest through the Paris region into Burgundy, with some areas possibly reaching 41C.
Authorities in Paris moved to ease the impact on residents, with Deputy Mayor Emmanuel Gregoire ordering parks to remain open around the clock.
The Eiffel Tower and other Paris venues set up misting stations to cool crowds, among a raft of measures announced by national and local authorities to minimise risks.
More than 200,000 people across Europe died from heat-related causes over the last four years, and most of the fatalities were preventable, the World Health Organisation's Europe office said this month.
More above-average temperatures are expected this northern summer, which can cause heat exhaustion and life-threatening heat stroke.
WHO's Europe office called for countries and institutions to implement heat plans, such as opening cooling centres, or introducing breaks or flexible shifts that enable workers to stay out of the midday sun.
France's annual Music Day on Sunday is a particular concern for authorities.
The nationwide summer solstice celebration involves thousands of concerts in village squares, rave venues and Paris clubs, bringing communities together and increasingly drawing international visitors.
The national weather agency Meteo France warned the heat would persist into next week, describing it as comparable to major episodes in 2003 and 2019.
The government ordered organisers of music day events to limit alcohol use to "preserve emergency services and allow medics to concentrate on taking care of the most vulnerable".
Authorities are notably worried about people living in the baking streets, and elderly people in nursing homes or isolated in their homes. About 15,000 older people died in a 2003 heat wave that became a reckoning for France.
The government announced on Saturday reinforced wildfire readiness and ordered tightened surveillance of water supplies to France's many nuclear reactors.
Schools will only be closed as a last recourse, the government said, though end-of-year exams held in the afternoons may be delayed until the following morning or otherwise rearranged.
Prime Minister Sebastien Lecornu convened a government heat crisis meeting on Saturday and plans another one on Sunday, in the face of what the national weather service called a "widespread, long-lasting and intense" hot spell.
Lecornu ordered government ministers to plan for better adapting France to heat waves in the future - including "via air conditioning, if necessary".
Scientists say climate change is making heatwaves more frequent and intense across Europe, raising the risk of health emergencies and economic disruption during the summer months.
