Nearly three quarters of Venice was flooded and tourists swam in St Mark's Square as a wave of bad weather swept through northern and central Italy.
Shops, homes and historic palaces filled with water and authorities said 70% of the lagoon city was flooded.
High water in Venice reached 149cm, 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 under the water.
There was no immediate estimate of damage to the northeastern city.
In Tuscany, 23cm of rain fell in four hours, causing the Ricortola and Parmignola rivers to flood, according to the regional government and forcing the evacuation of 200 people from their homes.
"It has been devastating," said Roberto Pucci, the 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. If there hasn't been a death it's a miracle," he told Corriere della Sera newspaper.
Local media said dozens of people took refuge on their roofs after rivers burst their banks in central Italy.
Environment Minister Corrado Clini called for more funding to shore up Italy's weather defences.
Bad weather with torrential rain is due to continue through tomorrow, forecasters said.
It was the fourth time since 2000 that Venice had been hit by record high water, and the city's environment officer said the latest flooding was the result of global climate change.
A barrier to protect the city from repeated winter flooding, which has been planned for decades, is due to be finished by 2015.