Hundreds of firefighters and soldiers - backed by helicopters and fixed-wing aircraft dropping water - are battling the fires but they are too numerous and widespread to contain.
"The situation is tragic. Fires are out of control on many fronts," said a regional Athens governor, Yiannis Sgouros. more than 30,000 acres of land had been burnt in what he described as "an ecological disaster".
Many residents of the threatened towns and suburbs have fled by car, motorbike and on foot, but others have stayed behind to try to defend their homes.
Two children's hospitals, a summer camp and a psychiatric clinic have been evacuated.