Dressed in a priest's clerical clothing, the man accessed staff-only areas of the famous Duomo, separate from the areas open to tourists or the congregation.
He was even pictured speaking to nuns in the cathedral during his visit.
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When he was spotted by police officers, they asked him to show his ID, at which point the fake priest at first tried to run away.
Then he offered a cover story, allegedly claiming to be an Orthodox priest - but police quickly saw through this, partly due to the fact he was wearing the wrong kind of garments.
According to Milan-based Il Giorno, the man was from Monza and explained that he thought up the plan in order to avoid paying for the €3 entry ticket.
However, he said he had bought the priest's clothes shortly beforehand for €70 - clothes which were then confiscated by police.
Photo: Miguel Medina/AFP
Police officers escorted him out of the cathedral, accompanied by soldiers who are on duty at the site as part of Italy's anti-terror measures which have seen security ramped up at high-profile monuments.