Paris faced another sweltering day after temperatures in the French capital hit a June record of 40.9C on Wednesday.
Britain also logged its highest temperature for June on Wednesday, reaching 36.1C in southern England as an early summer heat dome hovered over much of Western Europe.
France implemented a number of measures against heatwaves after one in 2003 caused nearly 15,000 excess deaths, with the elderly the hardest hit.
This time around, younger people were a particular concern.
"The profile of people currently facing health risks is not necessarily what one might expect - that is, the most vulnerable, because they are closely monitored and well informed," Emmanuel Gregoire, the mayor of Paris, said on broadcaster TF1.
"Rather, it's people aged between 50 and 70 who are generally in good health, but who think this is just a normal period and continue going about their usual activities as if nothing has changed. Really, protect yourselves."
At least 48 people have died in France from drowning since the start of the heatwave while trying to cool off, authorities said, and two young children were killed by heat in a car.
Air-conditioning remains relatively rare in Europe but Asian makers of air-conditioners, such as Samsung Electronics, Midea and Mitsubishi Electric, are enjoying a boom in sales, with strong demand from countries such as France, Spain and Italy.
The heatwave is being driven by a weather pattern known as an Omega block, pushing temperatures as much as 18C above normal.
The phenomenon resembles the shape of the Greek letter Omega, with a bulbous middle trapping in heat over regions for extended periods, with cooler weather on its fringes.
Heatwaves and storms are being intensified by climate change.
In Italy, Health Minister Orazio Schillaci called a meeting to address the risks after Italian media reported that five people had died on Wednesday from heat-related incidents.
France's weather agency said an end to the heatwave was in sight, with temperatures set to gradually decrease on Friday.
However, Germany, Austria and Italy looked set for a torrid weekend.
Germany's National Meteorological Service issued extreme heat warnings for a large area of western Germany, with temperatures expected to rise to 38C on Thursday and 41C on Friday and Saturday.
In neighbouring Austria, the national weather service issued a red heat warning for the northeast of the country, including Vienna, at the weekend, when the temperature in the capital could hit a record 40C.
The hottest conditions for Italy are also forecast between Saturday and Monday, with temperatures reaching up to 40 C - well above seasonal norms - particularly across northern plains and inland central areas.
