Italian police arrested three Kosovars sympathetic to Islamic State on Thursday on suspicion of operating a jihadist cell that had discussed blowing up the historic Rialto bridge in Venice.

A minor from Kosovo was also was detained to prevent him from interfering with the probe, police said. All had papers allowing them to live legally in Italy.

Up to three other Kosovars were also under investigation over their ties to those arrested, Adelchi D'Ippolito, Venice's chief prosecutor, told a news conference streamed online.

Police had been monitoring the group since last year. They tapped their phones, bugged their apartments and followed their online communications, and what emerged was "disturbing and worrying", D'Ippolito said.

The group had expressed an unconditional support for Islamic State, he added.

Those arrested had celebrated after a deadly March 22 attack in London, D'Ippolito said, adding that one had commented: "Considering how many infidels there are in Venice, we could get to heaven right away by putting a bomb on the Rialto bridge."

D'Ippolito said all the detained lived in the historic center of Venice, an ancient city in the north of Italy famous for its many canals.

In a statement, city mayor Luigi Brugnaro thanked police for "breaking up a dangerous and active jihadist cell in the center of the city."

Police also carried out 10 raids overnight in Venice, one in the neighboring town of Mestre and one in nearby Treviso.