Venice was hit by the highest tide in more than 50 years late on Tuesday, with tourists wading through flooded streets to seek shelter as a fierce wind whipped up waves in St Mark's Square.
The exceptionally intense "acqua alta," or high waters, peaked at 1.87 metres (six feet) as the flood alarm sounded across Italy's iconic city of canals, the tide monitoring centre said.
"We're currently facing an exceptionally high tide. Everyone has been mobilised to cope with the emergency," Venice's mayor Luigi Brugnaro tweeted.
Only once since records began in 1923 has the water crept even higher, reaching 1.94m (6.3 feet) in 1966.
As emergency services took to the canals to survey the damage, Italian media reported that a 78-year old man had been electrocuted as the waters poured into his home.
The coast guard laid on extra boats to serve as water ambulances.
Tables and chairs set out for aperitifs bobbed along alleyways in the dark, as locals and tourists alike waded through the streets, the water slopping over the top of even the highest boots.
Water taxis attempting to drop people off at the glamorous and historic hotels along the Grand Canal discovered the gangways had been washed away and had to help passengers clamber through windows.
"It will be a long night," Brugnaro tweeted, saying that as the water level began to drop again "the fears of a few hours ago are now being replaced by an assessment of the damage done".
He said he would declare a state of disaster for the city.