For the first time a system of 78 mobile floodgates has been tested in Venice, after years beset by delays and corruption.
The 1.5km (one-mile) Mose system of yellow dams was a “powerful project that has taken years to complete”, said Prime Minister Giuseppe Conte.
Venice was hit by the worst floods in half a century in November 2019.
Environmental protesters took to the lagoon on Friday, saying the barriers would damage the area.
Critics argue the sluice-gate system is 10 years too late. Work on the Mose project started in 2003, even though it was designed in the 1980s. It has gone three times over its original budget and resulted in the arrest of dozens of officials, the BBC’s Quentin Sommerville reports from Rome.
The coronavirus pandemic has brought tourism to a halt and stopped cruise ships from entering the lagoon but rising seas in the Adriatic continue to threaten Venice. Last autumn, more than 80% of the city was flooded.
The prime minister and three government colleagues visited the control roo