Pope Francis washes the feet of prisoners on Maundy Thursday
The Christian ritual takes place on the Thursday before Easter to commemorate Christ's Last Supper.
Thousands of pilgrims and tourists are arriving in Rome to attend ceremonies during the holy week ahead of Easter.
In a homily, the Pope earlier urged priests to do less "soul-searching" and engage more with parishioners.
"It is not in soul-searching... that we encounter the Lord," he told hundreds of cardinals, priests and bishops in St Peter's Basilica.
"We need to go out... to the outskirts where there is suffering, bloodshed, blindness that longs for sight and prisoners in thrall to many evil masters."
Worshippers should "leave Mass looking as if they had heard good news", he added.
Gesture of humility
During Thursday's intimate service at Casal del Marmo, the Pope washed and kissed the feet of 12 young detainees to replicate the Bible's account of Jesus Christ's gesture of humility towards his 12 apostles on the night before he was crucified.
The 12 inmates included two girls, one Italian Catholic and one of Serbian Muslim origin, local prison ombudsman Angiolo Marroni said ahead of the ceremony.
Some of the prisoners volunteered to have their feet washed, while others were given an invitation to help them overcome their embarrassment, the Catholic News Agency quoted the prison chaplain as saying.
The pontiff told inmates that Jesus had washed the feet of his disciples in a gesture of service, Vatican Radio reported.
"If the Lord has washed his disciples' feet, you should do the same," he is quoted as saying.
After the ritual, Pope Francis gave Communion to the inmates and prison workers.
In total, around 10 girls and 40 boys from different nationalities and diverse religious confessions were taking part in Thursday's Mass.
The new leader of the world's 1.2 billion Roman Catholics has brought a new sense of simplicity to the Vatican, reports the BBC's David Willey in Rome.
He has broken with tradition for the foot-washing ceremony, which is normally performed on lay people in one of Rome's basilicas.
Pope Benedict XVI visited the Casal del Marmo centre in 2007, but not for the Holy Thursday Mass. Only for the first two years of his pontificate did he perform the feet-washing himself, after which the task was delegated to priests.
Easter is the most important festival in the calendar of the Catholic Church.
On Good Friday evening the Pope will carry a wooden cross and pray at a ceremony at Rome's ancient amphitheatre, the Colosseum, commemorating Jesus' crucifixion.
On Saturday evening Pope Francis will celebrate the main Easter Vigil Mass in St Peter's Basilica.
And on Easter Sunday morning, the new Pope will deliver his first "Urbi et Orbi" message to the city of Rome and to the world.
During his inaugural general audience on Wednesday, Pope Francis called for an immediate political solution to the conflict in the Central African Republic after last weekend's coup. source