Pope Francis has urged mafia members to stop blaspheming against God and to embrace love and service.
He was paying homage to Father Giuseppe Puglisi who was shot dead by the mafia in Sicily 25 years ago.
The Pope warned the "brothers and sisters" that they could not "believe in God and be Mafiosi" - a reference to the fact that many mafia members are regular worshippers.
On Saturday morning, the Pope addressed the faithful at Palermo's Piazza Europa and will later visit Puglisi's parish in the working-class neighbourhood of Brancaccio.
The priest was renowned for working with young people to keep them away from drugs and the mafia.
The Pope said Sicily needed "men and women of love, not men and women 'of honour'", referring to the code mobsters use to define themselves.
"A person who is a Mafioso does not live as a Christian because with his life he blasphemes against the name of God," Pope Francis said, before pleading: "Change, brothers and sisters! Stop thinking about yourselves and your money... Convert yourselves to the real God."
Father Puglisi was shot dead on the doorstep of his home on his birthday, 15 September, in 1993.
His murder came at a time when mafia violence against opponents of their influence was of particularly grave concern.
Puglisi was declared a "martyr" who was "killed by hatred of the faith" by Pope Francis' predecessor Benedict XVI in 2012. He was beatified five years ago.
Beatification is the penultimate step to sainthood in the Catholic Church.
Last month, Pope Francis visited Ireland where he asked for forgiveness for members of the Catholic Church who "kept quiet" about clerical child sex abuse.
The Catholic Church faces ongoing questions about child abuse around the world.