Flash floods turned roads into raging torrents of mud and debris outside Athens on Wednesday, killing at least 13 people, inundating homes and businesses and knocking out a section of a major motorway.
There were fears the death toll could rise further as rescue crews searched for potentially missing people in flooded homes and streets on the western outskirts of the Greek capital.
The flooding came after a severe overnight storm brought driving rain to the area.
Roads turned into muddy rivers that carried away vehicles, tossing them into piles on roadsides and against fences and buildings.
Several walls from gardens and low buildings collapsed, filling the streets with rubble.
Eleven of the people killed - four women and seven men - were found in or near Mandra, a small town on Athens' fringes that was hardest-hit by the flood.
The coast guard recovered the bodies of two more men believed to have been swept out to sea by the flood.
As the floodwaters charged toward the sea, they carried debris that sank fishing boats in a small harbour.
Several people were being treated in a hospital for various injuries, including hypothermia.
The fire department said it had received more than 600 calls for help pumping water out of buildings and rescuing people trapped in vehicles and homes. It said it had deployed 155 firefighters with 53 vehicles.
A section of the motorway between Athens and Corinth was completely knocked out. Cars, trucks and buses were trapped in an inundated underpass.
Judicial authorities ordered an immediate investigation into the deaths and material damage.
Investigators would be looking into whether factors such as shoddy or illegal construction might have contributed to the severity of the flooding.
Local authorities shut schools in the areas of Mandra, Nea Peramos and Megara, while the fire department appealed to the public to avoid the area unless absolutely necessary in an effort to reduce traffic.
More hazardous weather was predicted for large parts of Greece later on Wednesday and in coming days, with storms predicted for western areas and for parts of the capital.
The deaths on the edges of the Greek capital came a day after authorities declared a state of emergency on the small Aegean Sea island of Symi due to torrential rainfall there that flooded homes and shops, swept vehicles into the sea and cut power after the local power station was flooded.