For the first time perhaps since the fall of the Roman empire, a group of centurions faces prosecution for mounting an assault on brother officers just a few paces from the Forum.

Unknown to the attackers, their fellow "centurions" were undercover police officers sent to investigate claims of racketeering and fraud in the shadow of the Colosseum. Dozens of modern-day Romans dressed as centurions or gladiators make a living by posing for photographs alongside tourists in return for tips and by enticing them onto tours in exchange for payments from the organisers.

But the business has been plagued by complaints from holidaymakers of centurions resorting to threats, and allegations that it is a "closed shop" from which outsiders are rigorously, and sometimes brutally, excluded. The three men arrested on Wednesday have been accused of assault and risk additional charges of criminal conspiracy, according to the Corriere della Sera newspaper.

They were among 30 people arrested as a result of the investigation. Others included tour guides and the owners of tour agencies.

The officers posing as centurions were reportedly approached and threatened in the Piazza Venezia by the three men who were subsequently arrested. An argument broke out, and swords – albeit wooden ones – were soon cleaving the air.

Unknown to the trio, a party of street cleaners at work nearby was also made up of undercover police officers. On seeing their colleagues attacked, they sprang into action and, watched by bemused tourists, clamped handcuffs on the aggressors.

The police operation was launched after four complaints from tour agencies and two more from Italian tourists who said they had been tricked by a costumed tout into paying for a tour of the Vatican museums that never materialised and then threatened by him when they demanded their money back.

Theirs was merely the latest in a long line of allegations. Others have involved claims of centurions demanding up to €30 (£26) for a posed photograph.

One ruse allegedly involves offering to take a photograph of a tourist with his or her own camera, and then refusing to give it back until a substantial amount of money has been handed over.

In 2007, police arrested a fake centurion following a reported attack on an American man and a Chilean woman which landed both in hospital. The same year police said they had arrested 28 unlicensed phony centurions and charged them with "violating laws banning commercial activity in an archaeological area".

Four years earlier there was a fistfight outside the Colosseum between rival bands of costumed ancient Romans. Also in 2003 police arrested a self-styled gladiator for carrying a real sword.

The Rome authorities said in 2002 they would be licensing the centurions who hang around the Colosseum and other historic sites. There was talk of tests to show they spoke English, had good people skills and adequate general knowledge.

Those who passed were to be put on a list, given a badge and assigned a pitch. But they would have to abide by regulations concerning the authenticity of their costumes.

Alessio Di Porto, a 28 year-old former souvenir hawker, said, however, that the scheme had never materialised. "We've been waiting for years to be authorised", he told Corriere. "Unfortunately, [the council] has forgotten about us."

• This article was amended on 12 August 2011. The original caption said that the photo showed a phoney gladiator being arrested outside the Colosseum. this has been corrected.