Around 10 inches of rain fell on the port town of Livorno in two hours between midnight and 2am, causing rivers and streams to overflow.
On the city of Livorno website an official wrote: “The emergency is not over.”
And in a video posted on Twitter mayor Filippo Nogarin added: “Livorno has never been so devastated by bad weather.”
He also said: “The situation is very difficult, it’s critical. The city is on its knees.”
Weather forecasters had not predicted the violent cloudburst over central Italy.
One British holidaymaker returning from the wedding of Rhiannon and Michael Edge, from South-east London, at Castello di Gabbiano, said: “The weather was beautiful, hot and sunny, on Friday.
“It seems to have deteriorated in a matter of hours.”
Heavy rain has also hit Rome where Mayor Virginia Raggi asked citizens to stay at home.
A Serie A football match between Lazio and AC Milan in Rome was delayed for an hour because of the weather.