Francis, who has made defence of the poor a trademark of his papacy, forcefully defended their rights in his homily at a New Year's Eve vespers service for thousands in St. Peter's Basilica.
He denounced situations where the poor were made to feel like criminals and "forced to behave like mafiosi" to defend themselves.
Earlier this month, police arrested 37 persons suspected of being part of a "mafia-like" organisation that guided public contracts to people close to the alleged boss of the organisation, a right-wing extremist with long-time ties to Rome's underworld.
The Italian media have dubbed the investigation, which is continuing, "Mafia Capital"..
Some contracts involved running migrant centres and camps on Rome's poor outskirts. Investigators said funds were pocketed by corrupt city administrators and their criminal cohorts instead of being used to improve squalid conditions.
Francis is also the bishop of Rome, which is both the Italian capital and the centre of Christianity. Calling it "our city", Francis said: "We have to defend the poor, not defend ourselves from the poor. We have to serve the weak, not use the weak".
In the current scandal, prosecution documents include transcripts of phone wiretaps that showed how those who won contracts often profited from serious social problems.
"Have you got any idea how much you can make out of immigrants?" one of those arrested says, referring to subsidies for providing services at temporary camps for migrants. "Drug trafficking brings in less".
After the arrests, Rome's mayor, Ignazio Marino, ordered a review of city contracts and Prime Minister Matteo Renzi proposed tougher national laws against corruption..