Thousands of residents near Athens fled their homes on Monday, including in the historic town of Marathon, as wildfires crept closer to the capital despite "superhuman" efforts to contain the blazes, officials said.

Authorities ordered at least five more communities and two hospitals northeast of Athens to leave after eight nearby villages, including Marathon, were told to evacuate on Sunday.

Marathon's mayor said the town, which gave it name to the long-distance race that is the centrepiece of the Olympics, was facing a "catastrophe".

A 30-kilometre (20-mile) long wall of flames, more than 25 metres (80 feet) high, was moving towards Athens, the ERT public broadcaster reported.

Eight people have been hospitalised with respiratory problems and authorities opened the Olympic stadium in northern Athens to house those fleeing.

"Civil protection forces battled hard throughout the night, but despite superhuman efforts, the fire evolved rapidly," fire brigade spokesman Vassilis Vathrakogiannis said.

"At this moment it has reached Mount Pentelicus and is headed in the direction of Penteli," he added.

A children's hospital and a military medical facility in Penteli were evacuated at dawn, Vathrakogiannis said.

The summer wildfire season in Greece this year has seen daily blazes after the Mediterranean country recorded its warmest winter and the hottest June and July since reliable data collection began in 1960.

Temperatures around Athens are forecast to peak at 39 degrees Celsius (102 degrees Fahrenheit) on Monday, with wind gusts of up to 50 kilometres (31 miles) per hour.

Prime Minister Kyriakos Mitsotakis cut short his holiday and returned to Athens on Sunday because of the crisis.

More than 670 firefighters with 183 vehicles and 32 aircraft were battling the blaze, the spokesman said.

- 'Fire near you' -

"Forest fire near you. Follow the instructions of the authorities," said SMS messages sent to people in the Attica region, indicating in which direction to flee.

Marathon's 7,000 residents were told to head for the coastal town of Neak Makri.

"We are facing a biblical catastrophe," said Marathon's mayor, Stergios Tsirkas. "Our whole town is engulfed in flames and going through difficult times," he told the Skai television channel.

Firefighters on Sunday battled to put out 33 of the 40 blazes that had broken out in the past 24 hours.

But the force was battling seven more in the high summer heat, the fire brigade spokesman said Sunday.

"Everything is burning," said Giorgos Tsevas, a farmer in Polydendri village which was engulfed on Sunday.

"I have 200 olive trees there but now they are gone," the 48-year-old said.

Civil Protection Minister Vassilis Kikilias had warned Saturday that half the country was under a high-risk warning for fires due to high temperatures, wind gusts and drought conditions.

"Throughout the night, winds remained strong, creating dangerous situations. Unfortunately their intensity is expected to increase in the coming hours," Vathrakogiannis warned.

Scientists warn that human-induced fossil fuel emissions are worsening the length, frequency and intensity of heatwaves across the world.

The rising temperatures are leading to longer wildfire seasons and increasing the area burnt in the flames, according to the United Nations Intergovernmental Panel on Climate Change.