Greece has declared a national day of mourning after flash floods killed at least 16 people.

There are fears the death toll will rise as rescue crews continue to search for missing people after a severe storm overnight turned roads into muddy rivers causing extensive damage to property and cars.

The force of the water was so strong it knocked out a section of a major motorway in the western outskirts of Athens.

Of those killed, 11 - 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 flooding swept vehicles away and damaged buildings. Credit: PA

The coast guard recovered the bodies of two more men believed to have been swept out to sea by the flood.

Several people were being treated in a hospital for various injuries, including hypothermia.

The fire department said it had deployed 155 firefighters with 53 vehicles in an attempt to deal with more than 600 calls.

An investigation into the deaths and material damage has been opened and investigators would be looking into whether factors such as shoddy or illegal construction might have contributed to the severity of the flooding.

Further storms were forecast for area in the coming days.