Greece has been engulfed in some of the worst fires since 2007.
Government spokesman Dimitris Tzanakopoulos said 20 people have been killed and at least 69 other people have been hospitalised with injuries.
He added that many of them were in a serious condition.
Prime Minister Alexis Tsipras cut short a visit to Bosnia because of the crisis.
He said: “We will do whatever is humanly possible to control it.
“I’m concerned at the simultaneous outbreaks both east and west of Attica.”
He said more than 600 firefighters and 300 vehicles are working to extinguish the wildfires.
Regional Greek authorities declared a state of emergency in the eastern and western parts of greater Athens as fires fanned by winds raged through pine forests and seaside towns.
Regional governor Rena Dourou called the state of emergency in East and West Attica.
Buses, water trucks and machinery are being provided to help battle the fires.
Authorities are also investigating reports that up to 10 tourists went missing after sailing on a boat to escape.
Greece has called on the EU for assistance and said Cyprus offered firefighters to help them battle the blazes and Spain offered to contribute aircrafts capable of dropping water on the flames.
It is not clear how many people have been forced to evacuate.
The mayor of Rafina, Vangelis Bournous, said he thought 100 homes have been burned down.
Greek authorities urged residents of a coastal region west of Athens to abandon their.
The cause of the fire is currently unknown, but a dry winter created tinderbox conditions.
Wildfires are not uncommon in Greece.