More than 560 firefighters backed by volunteers, 17 waterbombing planes, and 15 helicopters battled the wildfire, which broke out at around 3 p.m. on Sunday near Varnavas, 35 km north of the capital.
By Monday morning, the fire had spread southward and was burning on several fronts, including the village of Grammatiko, the ancient town of Marathon, the seaside municipality of Nea Makri, and Mount Penteli just north of Athens.
Moreover, strong winds constantly changing the direction of the flames are making it difficult for emergency services to contain the blaze.
"Despite a hard battle and the superhuman efforts overnight, the fire spread very fast and has reached Mount Penteli," a fire brigade official said.
Firefighting aircraft paused operations overnight but resumed early Monday as police assisted with evacuations. Some residents spent the night in shelters, and at least three hospitals in Penteli were evacuated.
The fire, with flames reaching as high as 25 meters, spread "like lightning" due to gale-force winds, fire brigade spokesperson Vassilis Vathrakogiannis said on Sunday.
Prime Minister Kyriakos Mitsotakis rushed back to Athens from a weekend break on the island of Crete to oversee the response to the blaze, a government official said, with memories still fresh of a 2018 fire that killed 104 people in the seaside town of Mati, near the capital.
The country has been struggling with destructive blazes since May. While summer fires are common in Greece, extraordinarily hot and dry weather linked to climate change has made them more frequent and intense, according to scientists.
After its warmest winter on record and long periods of little or no rainfall, Greece is forecast to record its hottest-ever summer.
