Five people have been killed and several are missing after heavy rains caused flooding in Italy's northern Emilia-Romagna region, officials say.
About 5,000 people have been evacuated, and some had to be rescued from roofs by helicopter.
The authorities say 14 rivers have broken their banks, flooding 23 towns. The mayor of Ravenna says his city is now "unrecognisable".
The Emilia-Romagna F1 Grand Prix this weekend has now been cancelled.
Discussions on Wednesday between local authorities and organisers of the race at Imola concluded that the event could not go ahead.
Further rain is expected in the region over the coming days.
"It's probably been the worst night in the history of Romagna," Ravenna Mayor Michele de Pascale told Italy's RAI public broadcaster. "Ravenna is unrecognisable for the damage it has suffered."
The mayor of nearby Forli, Gian Luca Zattini, said his city was "on its knees, devastated and in pain", AFP news agency reported.
In the city of Cesena, residents climbed on the rooftops and waited to be rescued by helicopter or boat.
In Castel Bolognese, the mayor said the situation was "catastrophic", the BBC's Sofia Bettiza in Italy reports. Thousands of people living in single-floor homes needed to be rescued, the mayor added.
There is currently no electricity in the town, our correspondent adds. People are sheltering in gyms and schools across Emilia-Romagna.
Regional President Stefano Bonaccini urged residents not go near the rivers. "Those who live in areas close to watercourses should move to higher floors," he wrote in a post on Facebook on Wednesday.
Italian Prime Minister Georgia Meloni chaired a crisis meeting with civil protection officials.
In some areas it has not been possible to gauge the rainfall, because the amount exceeded what their instruments could record.
The flooding follows weeks of drought, which impedes the ground's ability to absorb rainfall.
