LATEST: More than 100 million Europeans suffer in temperatures above 35C

France and Spain, among the countries worst hit, began counting the toll from the extreme temperatures, including a three-year-old boy who was trapped in his family's car.

AFP calculations based on forecasts from the German weather service and 2025 population projections from the European Joint Research Centre indicated that more than 380 million people would face temperatures of over 30C.

READ MORE: Why Europe is warming faster than the rest of the world

The UN's climate chief Simon Stiell said the heatwave -- made worse by buildings and infrastructure unsuited to such temperatures -- "has the fingerprints of the climate crisis all over it".

"It's the latest price to pay for fossil fuel pollution baking our planet. Until humanity stops burning colossal amounts of coal, oil and gas, extreme heat will keep getting worse," he added.

The deputy director of the EU's Copernicus Climate Change Service, Samantha Burgess, said the hot weather was due to a "heat dome" of trapped air from north Africa in a low-lying high-pressure system, preventing cooler air from moving in.

"While heat domes are a natural weather phenomenon, anthropogenic climate change is making heatwaves more severe and more likely to reach record-breaking temperatures," she added.

Most of mainland France was under extreme heat alerts on Thursday, with some 63 million people out of a total population of 67 million facing temperatures of over 30C.

READ MORE: 'People are at risk': Europe urged to invest in healthcare as heatwaves intensify

The heat will also surpass 30C for 70 million people in Germany, 48 million in Italy and 38 million in the UK, with high temperatures also in Belgium, Luxembourg and the Netherlands.

Temperatures are expected to fall in western Europe from Friday but eastern Europe was on red alert as temperatures climbed into the weekend.

Cooling off

In Spain, where new temperature records have been set for June, the MoMo monitoring system of mortality rates said 212 deaths between Sunday and Wednesday could be linked to the heat.

Three deaths in northern France's Pas-de-Calais region were "likely" caused by the heat while a three-year-old boy was found dead in a car in the suburbs of Paris, where temperatures topped 40C on Wednesday.

The boy climbed into his family's unlocked car while he was supposed to be napping, and was found unconscious, a public prosecutor said.

Two other children have died in similar circumstances this week.

Dozens of people, from couples to families with babies, decided to sleep in hammocks and on camping mats at the Buttes-Chaumont park in Paris to try to beat the heat.

"We've got everything we need, really. And quite a few people hang out here, so there's a good atmosphere," Maissame Decosse, 26, told AFP.

"It's better to be here than indoors."

Elsewhere, locals flocked to the Canal Saint-Martin in the north of the city, some with inflatables, after the authorities allowed swimming due to the heat.

But in Brussels, residents complained about the lack of public swimming facilities in the Belgian capital, that made it hard for residents to cope as temperatures nudged 40C.

"It's honestly a joke for a city like Brussels with more than a million residents," said Paul Steinbruck, co-founder of the Pool is Cool organisation.

In Germany, where temperatures were in the high 30s and expected to hit 40C through the weekend, several outdoor events were cancelled.

Rail operator Deutsche Bahn also told customers to avoid travel due to a high risk of disruption from wildfires, heavy rain and thunderstorms.

'Nature is angry'

The effects of extreme heat -- from dehydration to heatstroke -- are a concern for those caring for the vulnerable, including the very young and old, and those with medical conditions.

"Dehydration is a high risk for elderly residents," said Shiny Mathappan, manager of the Kingsley Court Care Home in Hayes, where fruit juice and water were handed out to dementia patients in shaded areas of the garden.

"When they've got dementia, they forget to ask about whether they feel thirsty," she said.

The UK's Met Office weather service extended a red warning for extreme heat until Friday night for London and the surrounding area, Forecasts of 37-38C Wednesday and Thursday would be a new June record.

Kingsley Court resident Lucine Nazikian, 97, said she was not keen on the heat and the world needed to take it seriously -- or pay the price.

"Nature is angry with us because we destroy everything," she said.