Accompanied by heavy rainfall, a stormy weather front nicknamed Byron has reached Athens.
In many parts of the city, streets have turned into fast-flowing streams, accompanied by intense lightning activity.
The fire service has received calls in Elefsina, Aspropyrgos and Piraeus.
According to the Meteo meteorological servcice, significant rainfall has already been recorded in Attica, particularly in the western parts of the region.
By 4 p.m. Thursday, 71.4 mm of rainfall was recorded in Vlychada, 67.6 mm in Vilia and 33.8 mm in Salamina.
The storm swept full-tilt into Greece from the south on Thursday, hitting the country in both the west and the east, with strong winds and heavy rain causing floods and rockfalls.
On Zakynthos in the west, authorities urged residents to avoid unnecessary travel for the duration of the storm after streets flooded and several rockfalls were seen in Alykes and other parts of the southern Ionian island.
Similar problems have been reported from Mani in the southeastern Peloponnese, where the Fire Service was called in to help get two people onto safe ground after they were trapped in a flood and a warning was sent to residents via the 112 emergency service hotline advising them to stay indoors.
Falling rocks also caused the closure of a section of the national highway linking the towns of Sparta and Gytheio, both of which experienced serious flooding.
And even further east, on the Aegean island of Rhodes, authorities ordered that all schools remain closed on Friday after intense flooding was seen on Thursday and forecasts warn of more downpours to come.
Schools have also been ordered to close on Friday in other parts of the country, including Attica, after the Hellenic National Meteorological Service updated its emergency severe-weather bulletin on Thursday morning, warning of dangerous conditions in at least nine regions – including the capital – as Byron expands north.
This story has been updated.