"With all the unbelievers there are in Venice, you put a bomb under the Rialto and you go straight to heaven," one of the alleged jihadist plotters said in a wiretapped conversation, said Adelchi d'Ippolito, the Venice prosecutor in charge of the case.
"That was one the most worrying and alarming remarks we heard," he said at a press conference, revealing that the group had been under surveillance since last year.
The suspects were detained in an overnight sweep carried out after it was established that they had undergone "religious radicalisation", according to a police statement.
Raids were conducted at 12 locations in the historic centre of the city, which is a magnet for millions of visitors from around the world.
D'Ippolito said the suspects appeared to have been studying how to build explosives but did not have the necessary components for making a bomb.
"There was a lot of talk about unconditional support to ISIS (the Islamic State group). It wasn't just theory and dogma," d'Ippolito said of the wiretaps.
They were also envisioning moving on to "planning and projects", he said.
Interior Minister Marco Minniti praised the police for what he called "an important success in our terrorism prevention effort."
The Rialto Bridge is the oldest of the four bridges that span Venice's Grand Canal, first built at the end of the 12th century.
The current bridge, an arched stone construction which dates from the late 16th century, is one of the best-known landmarks in the floating city and its walkways are frequently packed with tourists.
It was the only way of crossing the Grand Canal on foot for the best part of three centuries.
According to media reports, the wiretap evidence against the suspects also includes recordings of them celebrating the attack outside Parliament in London last week and discussing their desire to join Islamist fighters in Syria.