The mercury nudged above 40C in parts of France on Friday – the hottest ever June day in 11 different areas – but could climb up to 42C on Saturday.
So fierce is the heat that the health ministry has activated a special helpline, while the Red Cross is distributing fresh water to the homeless in Toulouse amid fears of people dying in the scorching sun.
Care homes are being sprayed with water to keep them cool through the weekend and, in the Gironde department, all outdoor events have been cancelled.
“This is the earliest heatwave ever recorded in France,” said Matthieu Sorel, a climatologist at Meteo France.
It was, he added, a “marker of climate change” with “many monthly or even all-time temperature records likely to be beaten in several regions” over the weekend.
In Spain, forest fires have burned nearly 9,000 hectares of land in the northwest Sierra de la Culebra region already this week, forcing about 200 people to evacuate.
Another 3,000 people were evacuated from the Puy du Fou theme park in central Spain on Friday due to a fierce fire nearby.
In northern Italy, several towns have announced water rationing and the Lombardy region might declare a state of emergency as a record drought threatens harvests.
“As a result of climate change, heatwaves are starting earlier,” said Clare Nullis, a spokeswoman for the World Meteorological Organisation in Geneva.
She added: “What we’re witnessing today is unfortunately a foretaste of the future.”
The UK is enjoying a relatively cooler weekend after recording its hottest day of the year on Friday, when temperatures reached more than 30C in some parts of the country.