Prime Minister Alexis Tsipras said he had requested help from Europe to fight the fires and announced army and airforce units were being deployed to help douse the flames.
"All firefighting forces, in addition to the army and the airforce, are on alert," Tsipras told reporters.
The fire in Athens broke out on brush land on Mount Hymettus, near the northeastern district of Kareas, and later spread to the outskirts of residential areas.
A huge grey cloud billowed over the city, and a monastery and a children's shelter were evacuated.
More From This Section
Earlier in the day, four villages were evacuated in the Peloponnese peninsula, near the town of Monemvassia, where strong winds fanned a raging wildfire, local officials told state agency ANA.
"It's hellish right now, there are hundreds of pockets of fire," Iraklis Trichilis, mayor of the town of Monemvassia, had earlier told Skai television.
National civil protection chief Tassos Mavropoulos told the station that reinforcements from the entire Peloponnese region were being rushed to the area, where the operation was hampered by the wind and mountainous terrain.
A seventh aircraft was forced to make an emergency landing but the two pilots escaped with only minor injuries.
Fire department spokesman Nikos Tsongas warned there could be further damage to property.
"The winds are strong and are not expected to weaken. There could be further damage," he said.
A fire also broke out on the island of Evia but quickly brought under control, a fire department spokeswoman said.