Three-quarters of Venice is under floodwater as high winds and heavy rain left six people dead across Italy.
Tourists were forced to wade through the streets after the water rose by more than five feet (156cm) to the highest level since December 2008.
The emergency prompted officials to shut down the lagoon city’s water bus system and remove the raised walkways normally put out in flooded areas.
Footage on social media showed shop owners using water pumps to try to protect their goods and people standing knee deep in St Mark’s Square and on the canal paths near the Rialto bridge.
In one restaurant waiters wore waterproof boots to serve customers.
Regional governor Luca Zaia has warned that flooding could reach the levels of the disaster that struck the city in 1966.
Although Venice experiences high water levels several times a year, attempts to relieve the problem have been beset by delays, cost overruns and corruption scandals.
The Moses project, which began in 2003 and involves building series of underwater barriers in the lagoon, is still not complete.
Mayor Luigi Brugnaro said he had asked to talk with prime minister Giuseppe Conte to underline the urgency of the scheme.
The storm has also forced officials to close major tourist attractions in Rome, including the Colosseum and Roman Forum, as well as the ancient Roman city of Pompeii.
Two young people died south of Rome when a tree hit their car while another was killed when high winds toppled slender pine trees in the nearby town of Terracina.
A 21-year-old man was killed by a falling tree while walking in Naples and a woman died after being struck by debris blown off a building.
“Stay at home. Do not go outside for any reason,” Terracina Mayor Nicola Procaccini told residents.
Much of the country is under alert for flooding and the Interior Ministry urged officials in the affected regions to consider closing schools and offices again on Tuesday.
Additional reporting by agencies
