Flash floods in the western outskirts of Athens have turned roads into raging torrents of mud and debris, killing a number of people and knocking out a section of a motorway.
The coast guard said it recovered two bodies from the sea near the area of flooding.
Authorities said they believed the two men, one aged in his mid-fifties and the other around 80, had been swept into the sea by the flash flood. If confirmed, it would put the death toll at seven.
The fire department said it is searching for at least a further three people who were reported missing by their families.
The flooding came after severe weather overnight, with a storm bringing driving rain to the area.
Roads turned into muddy torrents that carried away vehicles, tossing them into piles on roadsides and against fences and buildings.
Several walls from gardens and low buildings collapsed, filling roads with rubble.
A section of the motorway between Athens and Corinth was completely knocked out, with floodwaters inundating an underpass, trapping cars, trucks and buses.
Firefighters using ropes rescued passengers from a bus and from cars trapped on the road.
Judicial authorities ordered an immediate investigation into the deaths and material damage caused by the flooding.
Investigators would be looking into whether factors such as shoddy or illegal construction might have contributed to the severity of the flooding.
Several people were transported to hospital for treatment of injuries, including two women suffering from hypothermia and a driver injured as he escaped his truck, which had become trapped by rising waters.
The five initial fatalities – two women and three men – occurred in the Mandra area in separate incidents.
The bodies of a woman and a man were initially found in their flooded homes while later two more men and one woman were found dead, two in the yards of local businesses and one near a road.
Firefighters received at least 340 calls for help to pump water from flooded buildings and rescue people trapped by the rising waters.
Local authorities shut schools in the areas of Mandra, Nea Peramos and Megara, while the fire department appealed to the public to avoid heading to the area unless absolutely necessary in an effort to reduce traffic.
More bad weather was predicted for large parts of Greece on Wednesday and in the coming days, with storms predicted for western areas and for parts of the Greek capital.
The deaths came a day after Greek authorities declared a state of emergency on the small Aegean Sea island of Symi after torrential rainfall there flooded homes and shops, swept vehicles into the sea and cut power after the local power station was flooded.
