Italian authorities declared a state of emergency in Sardinia this morning, a day after a cyclone struck the Mediterranean island, killing 18 people and leaving hundreds homeless.

Overnight, Cyclone Cleopatra dropped 450mm of rain in an hour and a half, causing rivers to burst their banks, sweeping away cars and flooding homes across the island.

"This is a national tragedy," said Prime Minister Enrico Letta, who called a meeting of the cabinet to discuss the crisis.

He said the declaration of a state of emergency would allow extra resources to be freed up more quickly to reach devastated areas, with swathes of the island covered in muddy flood waters that covered cars and left houses submerged.

The mayor of Olbia, the northeastern Sardinian town among the worst affected areas, said the sudden flooding had burst "like a bomb" with the same amount of water falling in an hour and a half as falls in the city of Milan in six months.

Gianni Giovanelli said houses across the area had been left half submerged by the floods and rescuers were still searching for possible victims.

"We've just found a dead child we had been searching all night for," he told SkyTG24 television.

Regional governor Ugo Cappellacci told SkyTG24 television that the town of Olbia in the northeast had been flooded, with several bridges down, and that there was a similar situation near the central town of Nuoro.

Among the dead was a police officer, who was killed when a bridge collapsed. Three of his colleagues were injured.

In another incident, a woman and her daughter drowned when their car was tipped on its side by the flood.

A family of four living in a ground floor flat in an area of Olbia were found in their home, apparently after being surprised by the sudden explosion of water