Violent storms lashed the Athens region, turning streets into torrents in western suburbs, as forecasters warned that more heavy rain was on the way this week.
Floodwater carrying debris swept through the streets of Mandra and the seaside village of Porto Germeno, west of the capital. Some areas recorded as much as 40 millimeters (1.6 inches) of rain in an hour, according to reports.
Heavy downpours also hit central Athens, the northern suburb of Drosia, and coastal areas to the south and east of the city.
No injuries were immediately reported.
Civil protection services were on alert, and authorities urged residents to take precautions.
The unstable weather is expected to last at least until Tuesday, with possible hail in some areas, particularly in eastern Greece, Crete and the eastern Aegean islands.
"Before this system leaves, another, stronger low-pressure system from central Europe will bring rain again on Wednesday, locally heavy, across almost the entire country," meteorologist Nikos Kanteres told news website iefimerida.gr.
Mr. Kanteres said the bad weather would continue Thursday before easing Friday.
The storms cap an unusually wet October in Greece. Flooding earlier this month hit Crete, the island of Evia, Rhodes and the town of Argostoli on Kefalonia.
Mandra is particularly vulnerable. In November 2017, flash floods there killed 24 people in one of Greece's deadliest weather disasters in decades.