Heavy rain and storms hit Attica on Sunday evening. In Mandra and Porto Germeno, where they were worst, streets turned into rivers and the water swept along debris, videos on social media show.
The unsettled weather will last until at least Tuesday, 27 October. Eastern Greece is at the centre of the storm system, and hail is possible in places.
Reports on social media say 40 millimetres of rain, or 40 litres per square metre, fell in an hour in some places. Civil protection services are on high alert. The General Secretariat for Civil Protection called on people to take extra care and follow the safety measures.
Rain fell hard and without a break across central Athens, from Gazi and Patisia to Neos Kosmos, Kesariani, Imittos and Dafni. The northern suburbs were hit hardest, especially Drosia.
The south coast got the same heavy rain, in Nea Smyrni, Alimos, Palaio Faliro, Agios Kosmas and Vari. In the west, it hit Korydallos and Chaidari. In the east, it fell from Ilioupoli, Koropi and Kalyvia to Markopoulo and Porto Rafti.
Meteorologist Nikos Kanteres said the change began on Sunday in western Greece with rain and local storms, and is spreading to the rest of the country. The cause is a low-pressure system from central Europe that brings cold air. More rain and storms are due on Monday.
Kanteres also said the weather service expects eastern Thessaly, eastern Central Greece and Crete to be hit hardest, and later the islands of the eastern Aegean Sea.
He said the system will remain on Tuesday, though the weather will improve somewhat in western and northern Greece. Before it clears, a second, stronger low-pressure system will arrive from central Europe. On Wednesday, 28 October, Greece's national holiday, it will rain in almost the whole country, heavily in places, and probably in Attica too. Western Greece will be the exception, and Thessaloniki may get a short break.
The bad weather will continue on Thursday, and conditions are expected to improve by Friday.
Temperatures will fall, but not as much as last week, the meteorologist added. October has been very wet overall this year. Days earlier, very heavy rain and floods hit Crete, Evia, Rhodes and Argostoli.