Wildfires broke out near Athens on Monday as a heatwave grips Greece, firefighters said, with 1,200 children evacuated outside a seaside resort.

The first blaze started in Kouvaras, 50 kilometres southeast of Athens. 

"It's a difficult fire, the winds are really strong" with gusts reaching up to 60 kilometres per hour," said Yannis Artopios, a firefighters' spokesman. 

Greece, along with Italy and Spain, has been gripped by a heatwave since last week, with temperatures reaching 44 degrees Celsius in the centre of the country.

The domestic press agency ANA said a forest fire had also flared in strong winds near the Isthmus of Corinth close to the popular beach town of Loutraki.

"We have saved 1,200 children who were in the holiday camps" threatened by the fires near Loutraki, said the town's mayor, Giorgos Gkionis.

Seven water-bombing aircraft, four helicopters and 150 firefighters backed by colleagues from Romania were fighting the flames on two fronts, local media reported.

The blaze burnt quickly through brush and spread south toward the Attica region and the resorts of Lagonissi, Anavyssos and Saronida.

Several homes were burned in the area, according to footage by public broadcaster ERT, and an equestrian centre was evacuated.

Thick smoke blocked traffic on the roads around Kalyvia.

The authorities told residents to leave the area and monks were evacuated from a local monastery.

Artopios said no one had so far been reported trapped by the flames.

In Athens, where the mercury hit 39 degrees on Saturday, the Acropolis, one of Greece's top tourist attractions, closed during the hottest hours for three consecutive days to Sunday.

Temperatures are forecast to drop in Greece some 2C-4C by Wednesday, with a new heatwave to follow from Thursday and local highs of 43C, according to the EMY national weather service.

"We are in the middle of the period of fighting fires and the conditions expected will be particularly difficult and favour forest fires," said Artopios.