Temperatures in Spain are expected to hit 44 degrees Celsius, while France mobilized the emergency services, as Europe struggles with unusually hot weather and an enduring drought.
Spain is expecting extreme heat between Tuesday and Thursday, just three weeks after the last hot period, the AEMET weather service reported.
Thirteen of the 17 country's autonomous regions posted yellow, orange or red alert levels, in unseasonably hot weather for July.
The centre of the country and the south-western part of the holiday island of Mallorca, where temperatures of up to 36 degrees were expected, were worst affected.
Urban dwellers sought out fountains, ice-cream parlours and air-conditioned shopping centres in an attempt to escape the heat.
The forecast for Madrid was for 39 degrees on Tuesday, rising to 41 on Wednesday and 42 on Thursday. A temperature of 44 degrees was predicted for the Tagus Valley to the south-west of the capital on Thursday.
AEMET is predicting that maximum temperatures will decline to between 25 and 35 degrees over the days ahead, but only along the Atlantic and Mediterranean coastlines.
AEMET spokesman Rubén del Campo said heatwaves were constantly increasing in frequency and severity, attributing this to human-generated climate change.
In France, temperatures soared to between 33 and 37 degrees Celsius, and in places up to 38 degrees, according to weather service Météo France.
The forecaster issued a heat warning for seven departments, saying some areas would see temperatures reach 39 degrees on Wednesday, with the following days set to be even hotter.
The authorities in Alsace issued a drought warning and called on the population to use water sparingly. However, the drinking water supply was not affected, they said.
On Tuesday, Prime Minister Élisabeth Borne mobilized several ministries, aid services and local authorities in order to protect the public amid the extreme temperatures.
The authorities are to ensure support for people working outdoors, the elderly, pregnant women and the homeless.
The Agriculture Ministry is to convene a national drought commission to discuss the rational use of water resources later this week.
Housing Minister Olivier Klein was instructed to ensure shelter for people in emergency situations.
The hot and dry conditions throughout Europe are increasing the risk of forest fires. The worst fire at the moment is raging in Las Hurdes near Spain's western border with Portugal, where some 400 residents have been told to leave their homes, RTVE state television reported.
