The Greek government has appealed to the European Union for assistance as wildfires, which have killed at least 27 people, continue to blaze.
The government has announced a state of emergency in two provinces
Emergency services have been overwhelmed - with more than 170 fires raging across the country.
The government has declared a state of emergency in the provinces of Lakonia and Messinia, in what the prime minister called a "tragedy".
A blaze flared in Athens itself overnight, but is now under control.
The fire, in the Filothei district north of the city centre, broke out in the early hours of Saturday, and it took more than 10 fire engines to put it out.
Friday was the deadliest day of a terrible summer of forest fires, a war of attrition against the flames that has now been raging for two months, says the BBC's Malcolm Brabant in Athens.
At least nine people are reported to have burned to death in their cars as they attempted to flee the flames in the western Peloponnese region.
The victims, driving near the town of Zahero, were surrounded by a wall of fire and could not break through.
Three firefighters were among a total of 21 deaths reported near the town.
A local prefect close to the scene described it as horrific.
"The situation is extremely dire... The speed with which this fire has been spreading is astonishing," said the mayor of Zahero, Pantazis Chronopoulos.
Prime Minister Kostas Karamanlis visited Zahero late on Friday, and told reporters: "We are living through an unspeakable tragedy today."
Six deaths have been confirmed in the seaside town of Areopolis in the Mani region of Greece's deep south. Hotels and several villages have been evacuated, fire officials said.
Scores of other people in the region have been taken to hospital with burns.
The overstretched fire services are being helped by the military.
Acting Interior Minister Spyros Flogaitis called on EU countries "to send any help they can".
The biggest fires are still raging out of control, whipped up by dry winds gusting up to gale force, which have hampered the efforts of water-dropping aircraft.
Some helicopters have managed to take flight and their crews have been praised for heroism in the most difficult conditions.
More than 100 fires are burning across Greece
"The helicopters are operating whenever the weather conditions permit... The pilots are making incredible efforts," fire department spokesman Nikos Diamandis said.
Our correspondent says the large number of deaths has transformed what had previously been seen as a predominantly ecological disaster.
There has been widespread public anger at the government's response, which many have criticised as inadequate and slow.
Mr Karamanlis is under considerable political pressure, as he faces an early general election in three weeks' time.