Venice declared a state of emergency on Wednesday after "apocalyptic" floods swept through the lagoon city, flooding its historic basilica and inundating squares and centuries-old buildings.

"These are the effects of climate change," the Venice mayor, Luigi Brugano, wrote on Twitter on Tuesday night.

"Venice is on its knees," Brugnaro added on Wednesday. "The damage will run into hundreds of millions of euros."

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According to Brugano, the tide from Monday evening hit 187cm, which is just short of the overall record of 194cm set in 1966.

He said schools on the city's islands would also remain closed on Wednesday as another high tide of up to 145cm was forecast for later in the morning.

Pictures of the historic city showed many iconic places, such as Saint Mark's Square and Saint Mark's Basilica, underwater.

It is the sixth time in 1,200 years that the basilica has been flooded, with four of those incidents happening in the last two decades.

Meanwhile, tourists were pictured wading through knee-deep water along the city's most popular routes, while others braved the flooding to sit outside the city's famous cafes.

"A tide at 187 cm is a wound that leaves indelible marks," Brugano wrote in a later post. "Now the government must listen to Venezia."

Venetians and local business owners have been asked to take a record of all the damages to their homes and businesses as a plan of action is written up.

Watch: Extended coverage of Venice flooding