Greece is battling a dozen major wildfires, including one threatening its third-largest city Patras, as a heatwave stoked blazes and forced the evacuation of thousands in southern Europe.

Spain, Portugal, France, Italy, the Balkans and Britain have this week wilted in high temperatures that fuel wildfires and which scientists say human-induced climate change is intensifying.

Thousands of firefighters backed by the army deployed across Greece, with fire service spokesman Vassilis Vathrakogiannis saying "the situation remains difficult" for "forces waging a tough battle".

"It's a cocktail of high temperatures, strong winds... and minimal humidity," the head of civil protection for Western Greece, Nikos Gyftakis, told public broadcaster ERT.

AFP journalists saw residents and firefighters backed by helicopters frantically attempt to douse flames in scrubland and forests outside Patras, with thick smoke reducing visibility.

Officials evacuated 12 children from a Patras hospital, Vathrakogiannis said. Eighty elderly people were removed from a retirement home, and local media footage showed the roof of a nearby 17th-century monastery ablaze.

Other fronts were burning on the popular Ionian tourist islands of Zante and Cephalonia and the Aegean island of Chios, scarred by a huge wildfire in June that ravaged more than 4,000 hectares.

Around 20 villages were evacuated in the western Achaia region on Tuesday, while the Greek coastguard said nearly 80 people were removed from Chios and near Patras.

Wildfires dominated the news in Spain, where flames have threatened a world heritage Roman mining site in the northwestern region of Castile and Leon.

Regional authorities said almost 6,000 people from 26 localities had been evacuated from their homes, while seven people had been admitted to hospital for burns, including four in critical condition.

Interior Minister Fernando Grande-Marlaska said late Wednesday that Spain had asked the European Union for help, including the deployment of two water-bombing planes.

He urged European partners to deploy the planes quickly given "weather forecasts" that could worsen the blazes, telling broadcaster Cadena Ser radio that Spain would also request firefighter deployments if necessary.

Neighbouring Portugal deployed more than 2,100 firefighters and 20 aircraft against five major blazes, with efforts focused on a fire in the central municipality of Trancoso that has raged since Saturday.

Strong gusts of wind had rekindled flames overnight and threatened nearby villages, where television images showed locals volunteering to help firefighters under a thick cloud of smoke.

"It's scary... but we are always ready to help each other," a mask-wearing farmer told Sic Noticias television, holding a spade in his hand.

A separate fire in a mountainous zone of Arganil in central Portugal smothered several villages with smoke, prompting the evacuation of elderly residents in particular. (AFP)