"Calls to the emergency services are on the rise nationwide. We are seeing the beginning of a clear impact of the heatwave. For us the worst is still to come," said Jerome Saloman, head of national public health.
'Hell is coming': European heatwave intensifies
Europe's record-breaking heatwave was forecast to intensify Thursday, with authorities on alert as temperatures threaten to surpass 40 degrees in some regions.
A forest fire in Spain raged out of control on Thursday amid a Europe-wide heatwave, devouring land despite the efforts of hundreds of firefighters who worked through the night, local authorities said.
The blaze broke out on Wednesday afternoon in Torre del Espanol in the northeastern region of Catalonia and by Thursday it had destroyed more than 4,000 hectares (10,000 acres), the region's interior minister Miquel Buch said.
"The difficulties are such that we can't talk about a fire that is under control or in the extinction phase, but rather that we're at a moment when the blaze is getting bigger," he told Catalan radio.
In a statement, Catalonia's regional government warned the fire could eventually devour 20,000 hectares in what presented an "extreme risk."
The stifling heat has elsewhere prompted traffic restrictions in France and fanned debate in Germany over public nudity as sweltering residents stripped off.
Meteorologists blame a blast of hot air from northern Africa for the heat this week, which has already set new records in Europe for June.
Exceptional for arriving so early in summer, the heatwave will on Thursday and Friday likely send thermometers above 40 degrees in France, Spain and Greece.
In Spain, hundreds of firefighters and soldiers, backed by water-dropping aircraft, battled on Wednesday to put out a wind-fuelled forest fire that erupted in Torre del Espanol in the northeastern region of Catalonia.
The worst is expected on Friday when 33 of the 50 Spanish provinces face extreme temperatures, which could reach 44 degrees in Girona.
"Hell is coming," one Spanish TV weather presenter tweeted Wednesday.
In France, temperatures "unprecedented" for their timing and intensity - since 1947 when detailed surveys started - were expected to reach at least 39 degrees over two-thirds of the country, said weather service Meteo-France.
Health official Jerome Saloman said the impact of the extreme heat was starting to be felt in France, with an increase in weather-related calls to emergency medical services.
Some schools are expected to close Thursday and Friday, while several cities - including Paris and Lyon - restricted traffic to limit a build-up of air pollution.
French authorities were taking no chances after the August 2003 heatwave was blamed for 15,000 deaths in the country, with television and radio broadcasts issuing warnings.
In Greece, where around 100 people died in last year's deadly fires at the Mati coastal resort, hospitals and officials were on red alert with temperatures of around 45 degrees nationwide.
Vintage wine year
Scientists warn that global warming linked to human fossil fuel use could make such scorchers more frequent.
"Global temperatures are increasing due to climate change," said Len Shaffrey, professor of climate science at the University of Reading.
"The global rise in temperatures means the probability that an extreme heatwave will occur is also increasing."
But French winemakers said the hot weather was more than welcome as it could produce a superior vintage.
"Two of three days of heatwave in Bordeaux at this time, it's magic!" Philippe Bardet, head of the Bordeaux Wine Council, told AFP.
Temperatures above 40 degrees would help burn off any of the mildew caused by residual damp, which is "very, very good for quality", Mr Bardet said.
The 70-year-old record for the highest temperature recorded for June was beaten in Germany as 38.6 degrees was recorded in Brandenburg, the German Weather Service (DWD) confirmed Wednesday.
Meanwhile, police in Brandenburg cautioned a naked man for driving his moped wearing only his helmet and sandals.
And in Munich, security guards ordered a group of women sunbathing topless on the banks of the river Isar to cover up.
The move backfired, according to the Munich daily Sueddeutsche Zeitung, which quoted another sunbather as saying that she and others took their tops off "out of solidarity".
It said an urgent motion was introduced in a city council meeting Wednesday to allow topless bathing.
Scores of people have drowned in Poland and Lithuania as they tried to cool off in lakes and rivers, authorities said.
The Polish weather institute IMGW said the country's highest ever June temperature was recorded on Wednesday in the southwest: 38.2 degrees.
The Czech Hydrometeorological Institute also recorded the country's highest June temperature: 38.9 degrees in the northern town of Doksany.
In Belgium, vastly different temperatures were expected with 19 degrees on the north coast and 33 degrees in the south, according to broadcaster RTBF.