France, Italy, Portugal and Spain have been sweltering for several days as the mercury climbed to 44 degrees Celsius (111 Fahrenheit) in some places.

"This is unprecedented," Agnes Pannier-Runacher, France's ecology transition minister said as a record 84 of the nation's 96 mainland departments were placed on the second-highest "orange' heat alert.

Only a small sliver of the country in the northwest was not sweltering, according to the Meteo France weather service, which said the heatwave was due to peak on Tuesday and Wednesday.

The summer's first major heatwave has seen authorities in the countries along the Mediterranean's northern coast urging people to seek shelter.

Ambulances stood on standby near tourist hotspots as experts warned that such heatwaves, intensified by climate change, would become more frequent.

Firefighters were on standby after blazes broke out Sunday in France and Turkey, fed by the heat and strong winds.

Already last week, Greek firefighters had to battle a forest blaze on the coast south of Athens that forced some evacuations.

- 'Not normal' -

Spain's weather service AEMET said temperatures in Extremadura and Andalusia, in the south and southwest, had reached up to 44C Sunday.

In Madrid, where temperatures approached 40C, 32-year-old photographer Diego Radames told AFPTV: "I feel that the heat we're experiencing is not normal for this time of year.

"As the years go by, I have the feeling that Madrid is getting hotter and hotter, especially in the city centre," he added.

In Italy, 21 cities across the length of the country were on high alert for extreme heat, including Milan, Naples, Venice, Florence, Rome and Catania.

"We were supposed to be visiting the Colosseum, but my mum nearly fainted," said British tourist Anna Becker, who had travelled to Rome from a "muggy, miserable" Verona.

Hospital emergency departments across Italy have reported a 10-percent increase in heatstroke cases, according to Mario Guarino, vice president of the Italian Society of Emergency Medicine.

"It is mainly elderly people, cancer patients or homeless people, presenting with dehydration, heat stroke, fatigue," he told AFP.

- 'More frequent, more intense' -

Several areas in the southern half of Portugal, including Lisbon, were under a red warning until Monday night, said the Portuguese Institute for Sea and Atmosphere (IPMA).

Two-thirds of Portugal was also on high alert Sunday for extreme heat and forest fires -- as was the Italian island of Sicily, where firefighters tackled 15 blazes Saturday.

Scientists say climate change is stoking hotter and more intense heatwaves, particularly in cities where the so-called "urban heat island" effect amplifies temperatures among tightly packed buildings.

"The heat waves in the Mediterranean region have become more frequent and more intense in recent years," said Emanuela Piervitali, a researcher at the Italian Institute for Environmental Protection and Research (ISPRA).

"A further increase in temperature and heat extremes is expected in the future, so we will have to get used to temperatures with peaks even higher than those we are experiencing now," she told AFP.

- Invasive species -

The heat is also attracting invasive species, which are thriving in the more tropical climes.

ISPRA launched a campaign this week urging fishermen and tourists alike to report sightings of four "potentially dangerous" venomous species.

The lionfish, silver-cheeked toadfish, dusky spinefoot and marbled spinefoot are beginning to appear in waters off southern Italy as the Mediterranean warms, it said.

In France, experts warned that the heat was also severely hitting biodiversity.

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