Interior Minister Panos Skourletis, speaking on state television ERT, described the flooding as "a catastrophe, a tragedy".
It was not clear how many people went missing in the flash floods, particularly in the coastal area of western Athens, rescue services said.
Firefighters responded to more than 600 calls, winching vehicles and buses stranded in the high water with people inside, and saving many from their flooded homes.
The holiday region of Nea Peramos was hit hard, houses and restaurants were submerged, including around the village of Mandra where most of the deaths were reported.
"I have nothing left," a woman standing in front of her devastated home in Mandra told reporters.
"I only managed to save my children, but lost everything else, all of my home," another woman said.
The highway connecting Athens with the port city of Corinth was temporarily closed in two locations.
The floodwaters swept away trees, cars and anything else that was in their path. The debris then blocked the outflow toward the sea, creating more flooding.
Weather forecasters warned that the rain will continue in coming days, as a slow-moving low-pressure area lingers.
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