The man, identified as Marcello di Finizio, perched on a ledge some 80 metres above the ground, securing himself with a rope tied to a statue, an AFP journalist saw.
Di Finizio was demonstrating against a European Union directive that will see Italy auction off licences to run businesses on the seafront. He claims the measure threatens the survival of his beach club near Trieste in north-east Italy.
In other protests in recent years, Di Finizio managed to climb onto the Basilica's famed dome at the Vatican on four separate occasions.
In 2012, he remained perilously perched on the cupola for 24 hours before firefighters brought him down. At the time, he was protesting against austerity measures in the recession-hit country.
Earlier this year, he spent 80 days atop a crane in the city of Trieste.