Firefighters battled wildfires at a scale not seen for 20 years in Spain and southern France was placed on unprecedented red alert as much of western Europe sweltered in an extreme early-summer heatwave on Thursday.

With temperatures in northern Spain and southern France set to exceed 44C, governments urged their citizens to take the utmost precaution, warning that in some areas the worst was yet to come.

The conditions led officials to raise the French extreme heat alert to red. The alert, signifying a “dangerous weather phenomenon”, was the first since the system was introduced in 2004 following a 2003 heatwave that led to 15,000 premature deaths.

It was issued for the four southern départements of Hérault, Gard, Vaucluse and Bouches-du-Rhône. “All members of the public should be concerned, even those who are in good health,” the interior ministry warned.

In Spain, more than 500 firefighters and soldiers struggled to bring a huge forest fire under control in the Catalan province of Tarragona that has so far burned across 5,500 hectares (12,355 acres) of land.

Fifty-three people have been evacuated from their homes, five roads remain cut off and the civil protection authorities have advised people not to enter the area unless absolutely necessary. Hundreds of sheep have died in the smoke and flames.

“We’re facing a serious fire on a scale not seen for 20 years,” the region’s interior minister, Miquel Buch, said in a tweet. “It could burn through 20,000 hectares. Let’s be very aware that any carelessness could lead to a catastrophe.”

The head of the regional fire service said it was hard to be optimistic. “The terrain is complicated, which causes a lot of problems, and the weather conditions aren’t favourable,” David Borrell told Catalunya Ràdio. “That’s tiring us out and meaning we have to work hard to achieve our objectives.”

The 350 local firefighters battling the blaze have been joined by 221 specialists from Spain’s military emergency unit, as well as planes, helicopters, tractors and other heavy machinery.

As people across Europe sought out rivers, lakes, fountains and swimming pools to stay cool, authorities in Milan, in northern Italy, said a 72-year-old homeless man had died at the city’s main train station after falling ill due to the heatwave.

The French health minister, Agnès Buzyn, said the peak of the heatwave would not hit the south of the country before Friday and expressed irritation that despite an increase in the number of calls to emergency services, some people did not appear to be heeding health warnings.

“We see citizens who are quite irresponsible and continue to go jogging between midday and 2pm,” Buzyn said, also complaining of parents leaving their children in the car while shopping.