The Italian government declared a state of emergency after Cyclone Cleopatra swept across the island of Sardinia.
Thousands of people were forced to evacuate their homes after the floods hit, with bridges destroyed by gushing, muddy rivers and water levels reaching 3m in some places.
The death toll was 18, Italian authorities said on Tuesday.
"This is a national tragedy," Prime Minister Enrico Letta said.
The declaration of a state of emergency will allow resources to be freed up more quickly to reach devastated areas and care for the displaced, according to authorities.
The government also set aside $27 million in immediate emergency funds to help the rescue and clean-up work.
Olbia Mayor Gianni Giovanelli told news agency Sky TG24 early on Tuesday that the city had been hit hard by the "apocalyptic'' storm.
"We're at maximum alert," Giorgio Cicalo, an official from the Civil Protection Authority in Sardinia told RAI state television. "We haven't seen a situation as extreme as this, perhaps for decades. Especially because it's been across the whole island."
The local newspaper L'Unione Sarda said the dead included a policeman who was helping escort an ambulance when the car he was travelling in was involved in the collapse of a bridge in Dorgali.
The city of Gallura was hardest hit with six dead, the newspaper reported, including three people whose car was swept away in the collapse of another bridge.
In another incident, a woman and her daughter were drowned when their car was tipped on its side by the flood.