Torrential rains that exceeded forecasters' expectations triggered flash flooding that killed at least six people, with the worst-hit areas along the west coast.
A 3-year-old girl was the only survivor of a five-member family that perished in the port city of Livorno, in Tuscany, when they were trapped in their basement apartment by rising floodwaters. The family members killed included the girl's parents, 4-year-old brother and a grandfather.
The grandfather rescued the girl but he, too, drowned when he went back to try and save the boy, TV RaiNews24 reported.
Images of the house showed a courtyard covered with mud and debris, with neighbors looking on in shock as emergency workers moved in to recover the bodies.
The death toll could rise as at least two other people were known to be missing, Anna Marie Manzone, a provincial interior ministry official, told Sky TG24 TV. Sky reported that two elderly people died in a hillside hamlet outside of Livorno.
A seventh person was killed in a road accident but it was unclear if that was due to the weather.
"The city is literally devastated," said Livorno Mayor Filippo Nogarin, adding that "a crazy amount of rain" pummeled the area in just a few hours overnight.
Rome is also hit hard
Powerful winds brought down trees and parked cars were alternately submerged or carried away by floodwaters.
Residents in some parts of the city could be seen sweeping water out of their homes and piling up ruined furniture on the street, while in another courtyard a red delivery van had been flipped almost vertical by the rushing water.
Mayor Nogarin appealed for volunteers to help the city, which has a population of 170,000. Many travelers pass through the city to catch ferries to the islands of Elba and Sardinia. The flooding also interrupted train service along the Tuscan coast, along the Tyrrhenian Sea.
About 325 km (200 miles) to the south, in Rome, the deluge turned streets into fast-moving streams and forced underpasses and at least seven subway stations to close due to flooding.
"What's happening in Rome right now is unheard of... with a storm unleashing chaos. Once more the city has proved itself to be completely unprepared for rain," said Italian consumer association Codacons.
Barring an urgent need to go out, Mayor Virginia Raggi urged residents to stay indoors.
Thunderstorms lashed the city Sunday morning but potential hailstorms and strong winds were expected to last into Monday. The flooding was made worse by a summer-long drought that left the parched earth less able to soak up the deluge of water.
bik/rc (AP, AFP, Reuters, dpa)