Heavy overnight rain flooded towns near Athens on Wednesday, killing at least two people, trapping others in their homes and cars and shutting schools.
Roads in Nea Peramos, 27 km (17 miles) west of Athens, turned into fast-flowing rivers of mud, TV footage showed.
"Practically the entire town of Nea Peramos has flooded," Mayor Grigoris Stamoulis told Antenna TV. "I don't think we've ever seen such flooding before."
Stamoulis said efforts were under way to rescue people from cars and buildings but that he did not believe anyone was in immediate danger.
In the nearby town of Mandra, an elderly man and an elderly woman were found dead in their homes, the fire brigade said. Some residents were forced onto rooftops and balconies while 12 people were rescued from a bus on a bridge.
Authorities received at least 300 calls for help and more than 125 firefighters with 40 vehicles were assisting in the effort. A state of emergency was declared in the west Attica region, which includes Nea Peramos and Mandra, on Wednesday.
On Tuesday, Greece declared a state of emergency on the island of Symi after a storm swept cars into the sea, damaged homes and cut off electricity and water supplies.