The fire brigade worked throughout the night "but despite superhuman efforts, the fire evolved rapidly", spokesman Vassilis Vathrakogiannis said on Monday morning.
More than 500 firefighters and 150 vehicles have been deployed to help put out the flames.
He said the fire was heading to the village of Penteli, north-east of Athens, and at least three hospitals there have been evacuated.
Climate crisis and civil protection minister Vassilis Kikilias warned that extremely dangerous weather would continue. Greece has just experienced its hottest June and July on record.
Greek PM Kyriakos Mitsotakis returned to Athens on Sunday, cutting his holiday short, to deal with the crisis.
In Varnavas, the wildfire that started on Sunday afternoon had burned 100 sq km (38 sq miles) of land by 19:00 BST.
Varnavas and Grammatiko residents were instructed to evacuate in a message from the national emergency number.
Residents of historic town Marathon were told to evacuate towards the beach town of Nea Makri.
One resident from the village of Polydendri said "everything is burning".
"I have 200 olive trees there but now they are gone," said 48-year-old Giorgos Tsevas, .
"All the residents here have olive groves, or vines... All of them destroyed."
Another man who lives in Varnavas, Nikos Anagnostou, said the front of the fire was "really huge" and "uncontrollable".
"All of the olives are burned. And this year was a good year for oil."
Fire brigade spokesman Mr Vathrakogiannis said on Sunday the fire was quickly fanned to a huge size by winds of up to 8 Beaufort (39-46mph).Asthens
"We are appealing to all residents of the area. For their own safety and given that the fire is advancing rapidly, it is imperative that they heed the instructions of the authorities," Mr Vathrakogiannis said.
"At this time, the fire is advancing toward Marathon Lake with the forces against it being constantly reinforced, focusing on the flanks and dealing with flames that in some cases are higher than 25m," he added.
The strong winds were greatly hampering the work of the firefighting aircraft participating in the operation, with the fight against the fire expected to last through the night as the winds were expected to continue to be very strong.
Medical centres and hospitals in Athens remained on alert on Sunday evening. Parts of the capital were left under a cloud of brown smoke.
