The group demonstrated outside a gym owned by Roberto Spada, who was filmed headbutting TV reporter Daniele Piervincenzi, before attacking him with a baton.
Piervincenzi had been investigating Spada's alleged links to the far-right CasaPound movement and his nose was fractured in the attack.
“We are here to say a headbutt or even criticism will not stop us,” Dino Giarruso, a colleague of Piervincenzi, said at the protest in the coastal resort of Ostia, just outside Rome.
“Journalists are not exempt from reproach but that in no way justifies being assaulted when you are doing your job – it's unacceptable in a democratic country,” Giarruso added.
Piervincenzi was questioning Spada for a report for national broadcaster Rai about municipal elections, two years after the local council was dissolved due to mafia infiltration.
CasaPound, suspected of links to organised crime in the area, won eight percent of the first round votes.
Italian police arrested Spada on Thursday for assault aggravated by mafia-style violence, with prosecutors saying his behaviour was typical of methods used by organised crime groups to control territory.
He faces up to three years behind bars.
The Spada clan is notoriously violent. Seven members of the family were sentenced to a combined 56 years in jail in October, and Roberto's brother Carmine was ordered to serve 10 years last year for extortion and mafia association.