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Torrential rain has caused rivers to burst their banks and exposed problems with drainage systems across much of northern Italy. Authorities said a man died when he was caught by flooding of the Chiarone river near the town of Capalbio.
Nearly three-quarters of Venice was flooded, including St Mark's Square. Shops, homes and historic palaces filled with water.
Tuscany governor Enrico Rossi met officials from the Civil Protection Agency and asked for pumps and other emergency equipment from the neighbouring region of Emilia Romagna, which has also seen severe weather in recent days.
He asked the government to send army units to the towns of Grosseto, Arezzo and Siena after dozens of people were forced to leave their homes. Television pictures showed half-submerged cars and people wading in thigh-deep water along the streets.
"It has been devastating," said Roberto Pucci, mayor of Massa Carrara in Tuscany, one of the worst hit areas.
"I saw at least six bridges destroyed in the hills, floods, landslides, vineyards and olive groves swept away," he told Corriere della Sera newspaper.
The Italian Red Cross deployed 150 volunteers and was operating emergency food kitchens.
High water in Venice reached 149 cm (5ft), the sixth highest level since records began in 1872, forcing residents to wade through waist-deep water. Tourists in swimming costumes sat at cafe tables.
There was no immediate estimate of damage to the northeastern city. It was the fourth time since 2000 that Venice has seen record high water. The city's environment officer blamed climate change.
A barrier to protect the city from repeated winter flooding is due to be finished by 2015.
Flooding in northeastern Italy last year killed several people.
"There is an urgent need for a national maintenance and management programme for the regions," Environment Minister Corrado Clini told SkyTG24 television.