Spain's prime minister said "more than 500 people died" during a 10-day heatwave as Europe counts the cost of a record period of extreme temperatures.
Climate change protesters warned the scorching weather should be a wake-up call for the continent.
Spanish Prime Minister Pedro Sanchez said the "climate emergency is a reality".
Speaking against a backdrop of charred trees and burnt ground in the hard-hit northeastern Zaragoza region, he urged people to take "extreme caution".
Sanchez cited figures released by the Carlos III Health Institute, which estimates the number of heat-related fatalities based on the number of excess deaths compared to the average in previous years.
The institute has stressed these figures are a statistical estimate and not a record of official deaths.
Meanwhile Greek firefighters gained the upper hand in a battle against a wildfire raging for a second day in mountainside suburbs north of Athens that had forced hundreds of people to flee, an official said.
"For the most part the fire is in decline," fire department spokesman Yiannis Artopios told reporters.
Greece had been spared the blistering heatwave experienced in western Europe, but flames fanned by high winds were threatening the suburbs of Penteli, Pallini, Anthousa and Gerakas, home to tens of thousands of people.
"The fire was scorching our backs, we left in the nick of time. Had we stayed another 30 seconds it would have burned us," a Pallini resident who lost his car and shed to the flames told ERT television.
"The civil protection authority was late in alerting us," he said.
In France, firefighters brought twin blazes near the southwestern city of Bordeaux under control.
Temperatures of more than 40 degrees Celsius (104 degrees Fahrenheit) over previous days have spelled misery for millions and shattered heat records.
Cooler air swept in Wednesday, bringing relief to people from Portugal to Britain, but thousands of firefighters continued to tackle blazes that have broken out in multiple countries after months of drought-like conditions.
"Our assessment is generally positive. The situation improved overnight," French fire service spokesman Arnaud Mendousse told AFP from the southwestern Gironde region where two huge blazes have engulfed 20,600 hectares (50,900 acres) of tinder-dry forest since last week.
- Heat records -
The heatwave saw a new all-time record for Britain where the national weather service clocked 40.3C in eastern England, surpassing the previous high set in 2019.
Grassland fires erupted on Tuesday on the edge of London, with one forcing the evacuation of 14 people as farm buildings, houses and garages were consumed by the flames.
Sixteen firefighters were injured around the capital with two taken to hospital, the London Fire Brigade said.
"Yesterday was the busiest day for the fire service in London since the Second World War," the city's mayor Sadiq Khan told Sky News on Wednesday, urging the public to remain vigilant despite temperatures now falling.
Khan also accused Conservative leadership candidates vying to succeed Prime Minister Boris Johnson of ignoring "the elephant in the room" of climate change.
Climate demonstrators triggered a lengthy tailback on Britain's busiest motorway encircling London on Wednesday as they sought to highlight the need for greater action to reduce greenhouse gases responsible for global heating.
Members of the group Just Stop Oil climbed gantries over the M25 motorway, causing police to intervene.
"This is the moment when climate inaction is truly revealed in all its murderous glory for everyone to see: as an elite-driven death project that will extinguish all life if we let it," the activist group said in a statement.