"The problem was that he looked a lot like Karol Wojtyla. He was detained for usurpation of title which is a misdemeanour," a police spokeswoman told AFP.
"The cassock he was wearing has been confiscated," she added.
The man has been released and will have to pay between 154 and 929 euros ($200 and $1,200), depending on a decision by a judge, the spokeswoman said.
Another officer at the station where the 55-year-old man was taken quipped: "If he had been dressed like Tutankhamun nothing would have happened."
The performer has been working the main avenue leading up to the Colosseum in his white skullcap for weeks -- alongside Roman centurion imitators and other performers -- but police said they acted following an anonymous complaint.
A vendor selling miniature Colosseum souvenirs on a spot overlooking the Roman Forum said he saw the man being detained on Thursday.
"He was right here, near me. He was taken away by two officers in civilian clothes who were pretending to be tourists," he said - AFP