At least three forest fires erupted in Catalonia, including one near Baldomar around 140 kilometres (87 miles) northeast of Barcelona, that has burned nearly 1,000 hectares (2,470 acres) but could grow to 20,000 hectares before it is contained, the regional government said.
No evacuations have yet been ordered but people are being urged to remain in their homes.
Neighbouring Portugal saw its hottest May since 1931, with most scientists attributing the early season heat across Europe to global warming caused by greenhouse gas emissions.
Scientists say heat waves have become more likely due to climate change. As global temperatures rise over time, heat waves are predicted to become more frequent and intense and last longer, and their impacts more widespread.
‘Worst I’ve ever seen’
The heatwave crossed the Pyrenees into southern France on Tuesday and was set to hit most of the country by Saturday, when thermometers could reach 39C in Paris.
Most of France was on heatwave alert, including 12 departments at the highest level in the southwest, where the education ministry advised parents to keep younger students at home if possible on Friday.
“Be alert! Hydrate, stay in cool areas, and stay in touch with those close to you,” Prime Minister Elisabeth Borne said on Twitter.
Officials in Paris and other cities have also issued alerts over ozone pollution, which occurs when intense sunlight transforms carbon emissions into smog.
Paris police chief Didier Lallement has cut speed limits for the region by 10 km/h (6.2 mph) for Friday.
“I’m 86 years old, I was born here, but I think this is the worst heatwave I’ve ever seen,” Jacqueline Bonnaud told AFP at a shaded park in the southern city of Toulouse.
Surging use of air-conditioners and fans was forcing France to import electricity from neighbouring countries, grid operator RTE said, since many of the country’s nuclear reactors are offline to evaluate potential corrosion risks or for maintenance.
The intense heat is also lowering river levels, meaning some nuclear plants must reduce output because water used for cooling reactors is too hot to be returned to waterways without endangering plants and wildlife.
Spain, Italy and other countries have recently limited the use of air-conditioners to save energy, and French Energy Minister Agnes Pannier-Runacher told France 2 television that she was considering the same.
“Saturday will be the peak, with temperatures of 35 to 39 degrees across most of the country,” Tristan Amm, a Meteo France forecaster, told AFP.
Schools have stocked up on water and several have moved end-of-year exams to north-facing rooms, while some departments in the south have said classes will be cancelled on Friday afternoon.
Cities such as Bordeaux have also installed misting devices on the hottest squares and streets.
Rail operator SNCF has warned of potential delays as trains are forced to slow because the heat has deformed tracks or damaged electrical equipment.
“Our infrastructure suffers” in the heat, said SNCF regional director Thierry Rose, noting that track-level temperatures in Bordeaux had hit 52 Celsius (126 Fahrenheit) on Wednesday.