City mayor Luigi Brugnaro ordered the iconic square closed ahead of the latest sea surge of around 1.6 metres (over five feet) mid-morning as strong storms and winds batter the region - lower than Tuesday's peak but still dangerous.
Hotels, churches, shops and homes in the city of canals have been inundated by unusually intense "acqua alta", or high waters, which on Tuesday hit their highest level in half a century.
In Pictures: Venice left submerged as exceptional tide sweeps through canal city
The crisis, driven by bad weather, has prompted the government to release 20 million euros in funds to tackle the devastation.
The water was expected to reach 1.5 metres (five feet) mid-morning on Friday as strong storms and winds batter the region -- lower than Tuesday's peak but still dangerous, local officials said.
Undeterred, some tourists continue to visit the flooded St Mark's Square during breaks from the rain, snapping selfies in neon plastic boots.
Prime Minister Giuseppe Conte, who has called the flooding "a blow to the heart of our country", said late Thursday that a state of emergency had been approved.
Earlier that day he met Venice's mayor and emergency services before jumping in a speedboat to visit businesses and locals affected by the tide.
Photo: Filippo Attili/Palazzo Chigi press office/AFP
Residents whose houses have been hit would immediately get up to 5,000 euros in government aid, while restaurant and shop owners could receive up to 20,000 euros and apply for more later, he said.
Several museums remained closed to the public on Thursday.
As authorities assessed the extent of the damage to Venice's cultural treasures, such as St Mark's Basilica where water invaded the crypt, locals were defiant.
Many stopped for their usual coffees at flooded bars, drinking espresso while standing in several inches of water.
Austrian tourist Cornelia Litschauer, 28, said she felt mixed emotions seeing Venice's famous square half-submerged.
"For the tourists it's amazing, it's something to see. But for the people who live here it's a real problem," Litschauer said, cradling her white Chihuahua Pablo.
"It's strange. Tourists are taking pictures but the city is suffering."
The Locanda Al Leon hotel said its bookings had suffered from the international media coverage of the flood, with some guests cancelling their rooms after seeing images of Venice underwate
Tuesday's high waters submerged around 80 percent of the city, officials said.
READ ALSO: Venice flooding: St Mark's priceless mosaics drowned in sewage
Only once since records began in 1923 has the water crept even higher, reaching 1.94 metres in 1966.
Former prime minister Silvio Berlusconi arrived for a private tour of the damage sustained to the basilica, while rival leader of the Italian right Matteo Salvini was due to drop by for the same on Friday.
Many, including Venice's mayor, have blamed the disaster on global warming and warned that Italy - a country prone to natural disasters - must wake up to the risks posed by ever more volatile seasons.
"We need to be resilient and adapt. We need a policy that looks at the climate through completely different eyes," Environment Minister Sergio Costa said on Thursday.
Mayor Luigi Brugnaro has estimated the damage to Venice at hundreds of millions of euros.
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