Weather alerts were in place across Spain's Canary Islands, Italy, Cyprus and Greece, with the Greek authorities expecting temperatures to reach as high as 43 Celsius (109.4 Fahrenheit) or 44 Celsius on Friday or Saturday.
Authorities put an ambulance on standby near the archaeological site of the Acropolis in Athens, ready to provide first aid to tourists wilting in the heatwave, which Italy's Meteorological Society has named "Cerberus".
The European Space Agency (ESA), whose satellites monitor land and sea temperatures, said July could be a torrid month.
"Italy, Spain, France, Germany and Poland are all facing a major heatwave with temperatures expected to climb to 48 Celsius on the islands of Sicily and Sardinia – potentially the hottest temperatures ever recorded in Europe," it said.
The impact of extreme summer heat has been brought into focus by research this week that said as many as 61,000 people may have died in Europe's sweltering heatwaves last summer.
Governments and employers are under pressure to do more to protect workers exposed to the burning sun in the latest heatwave, named after the three-headed dog of the underworld in Greek mythology.
There are concerns about the impact on those working outdoors in Italy after a 44-year-old man who was painting road markings in the northern town of Lodi collapsed and died this week.
"Heat is a silent killer. So this is the main concern that people's lives are at risk," said climate scientist Hannah Cloke, a professor at England's Reading University.
"Certainly, we should immediately stop pumping greenhouse gases into the atmosphere," Cloke added, warning that some changes to the climate were already locked in.
SCORCHING EARTH FROM BALKANS TO SPAIN
Health authorities issued a top, red alert warning for 10 Italian cities for the next two days, including Rome, Florence, Bologna and Perugia.
In Greece, the government has ordered the suspension of work between 12pm and 5pm in areas where the risk from heat is very high, and also requested remote work for private sector employees with health conditions.
In the Balkans, beachgoers in the Croatian town of Nin smeared themselves in its medicinal local mud to protect themselves from the sun while 56 firefighters with 20 vehicles and three aircraft struggled to contain a brush fire near the Adriatic town of Sibenik.
Cloke said that the current heatwave was caused by hot air coming up from the Sahara, with the air mass then becoming lodged across parts of Europe.
Images captured by the ESA's Sentinel 3 satellites had measured the land surface temperature at more than 60 Celsius in the western Spanish region of Extremadura on Tuesday.
Weather forecasts and official records are based on the air temperature which is significantly lower than the land surface reading. The record European temperature of 48.8 Celsius was registered in Sicily in August 2021 and that figure could be exceeded.
"Next week there will be an even stronger heatwave than this one, some values in the central south will be really freaky," said Luca Lombroso, meteorologist from the AMPRO group in Italy.
"Between Tuesday and Wednesday in Rome and Florence we will probably exceed 40 degrees, which will also be approached in the north," he added.
Animals are also feeling the strain.
Italian farmers' lobby group Coldiretti said milk production was down by around 10% because cows eat less in the heat, drink huge quantities of water and make less milk.
