Italy issued hot weather red alerts for 16 cities on Sunday, with meteorologists warning that temperatures will hit record highs across southern Europe in the coming days.
Spain, Italy and Greece have been experiencing scorching temperatures for several days already, damaging agriculture and leaving tourists scurrying for shade.
But a new anticyclone dubbed Charon, who in Greek mythology was the ferryman of the dead, pushed into the region from north Africa on Sunday and could lift temperatures above 45 Celsius (113 Fahrenheit) in parts of Italy early this week.
"We need to prepare for a severe heat storm that, day after day, will blanket the whole country," Italian weather news service Meteo.it warned on Sunday.
"In some places ancient heat records will be broken."
Greece closed the ancient Acropolis during the hottest part of the day on Friday to protect tourists.
Italy's Health Minister Orazio Schillaci said people needed to take care visiting Rome's famous ruins.
"Going to the Colosseum when it is 43C (109.4F) is not advisable, especially for an elderly person," he told Il Messaggero newspaper on Sunday, saying people should stay indoors between 11 a.m. and 6 p.m.
DEHYDRATION
Besides the Italian capital, health alerts were in place from the central city of Florence to Palermo in Sicily and Bari in the southeast of the peninsula, while the temperatures also started to build further north.
"This is not normal. I don't remember such intense heat, especially at this time of year," said Federico Bratti, sunbathing at Lake Garda.
In Spain, forecasters warned of the risk of forest fires and said that it would not be easy to sleep during the night, with temperatures unlikely to fall below 25C (77F) across the country.
The heatwave will intensify from Monday, with temperatures reaching 44C (111.2F) in the Guadalquivir valley near Seville in the south of the country, forecasters predicted.
On the Spanish island of La Palma in the Canaries, meanwhile, at least 4,000 people had to be evacuated as a forest fire burned out of control following a heatwave, authorities said.
Europe's highest recorded temperature of 48.8C (119.8F), registered in Sicily two years ago, could be exceeded in the coming days, notably on the Italian island of Sardinia, meteorologists have said.
The heatwave has stretched across the Mediterranean to Israel, where Prime Minister Benjamin Netanyahu was hospitalised on Saturday suffering dizziness and apparent dehydration. He was discharged on Sunday.
"I ask you all, spend less time in the sun, drink more water, and may we all have a good new week," he said.
The United States was also in the grip of high temperatures, with nearly a quarter of the population under warnings for extreme heat, from the Pacific northwest, down through California, through the Southwest and into the Deep South and Florida.