More than 20 people have been killed in out-of-control forest fires near Athens, government spokesman Dimitris Tzanakopoulos said in a televised address early Tuesday.
Blazes were reported to the west and the east of the Greek capital amid searing summer temperatures of 40 degrees Celsius and high winds.
A state of emergency has been declared in the Athens metropolitan area.
Firefighters reported 90 people injured in the flames west of Athens, some of them seriously.
According to news reports early Tuesday, the official death toll could rise as rescue crews were discovering more bodies.
Greek Prime Minister Alexis Tsipras cut short a visit to Bosnia-Herzegovina to return to Greece. He suggested that some of the blazes might have been started by arsonists.
Tsipras ordered additional help from fire departments and emergency services in neighbouring regions and requested assistance from the military, Greek state radio reported.
Extreme drought and strong winds fuelled the flames near the Greek holiday destination of Kineta on Monday, with smoke clouds billowing 40 kilometres to the east, reaching Athens.
The area is densely wooded and dotted with holiday homes.
In the port town of Rafina, the flames have moved from the outskirts into the town centre, forcing hundreds of people to flee into the surf.