Prisoners at a youth detention center in Rome are preparing for a special guest Thursday afternoon: Pope Francis.

In one of many breaks with papal tradition since he was elected to lead the Catholic Church on March 13, the Argentine pontiff has chosen to celebrate the Mass of the Lord’s Supper at the center instead of the basilica of St. John Lateran where it is traditionally held.

The mass will take place in the chapel of the Casal del Marmo Penitential Institute for Minors, and Francis has made it his “express desire” that it be “very simple.” Pope Benedict XVI also celebrated mass there in 2007.

But most poignantly, the Pope will wash the feet of 12 detainees who will be chosen from different nationalities and religious confessions. The Vatican says about 10 girls and 40 boys will take part in the mass, saying the readings or the prayers of the faithful.

Christian churches throughout the world have a similar ceremony of foot washing on Holy Thursday, a tradition that derives from the gospel of John, in which Jesus washes the feet of his disciples as an example of love and service. "If I then, your Lord and Teacher, have washed your feet, you also ought to wash one another’s feet," Jesus says in the gospel. "For I have given you an example, that you should do as I have done to you."

But traditionally, priests have washed the feet of sub-deacons, clergy or laymen, as the ceremony is a memorial of Christ and his apostles. Church law states that only men can have their feet washed, so the Pope’s decision to perform the ceremony in a juvenile prison has caused controversy in some quarters.

The Vatican has been keen to point out that when he was Archbishop of Buenos Aires, Cardinal Bergoglio celebrated the mass in a prison, hospital, or hospice for the poor and marginalized, and washed the feet of 12 who lived there. “With this celebration at Casal del Marmo, Pope Francis will continue his custom, which is characterized by its humble context,” the Vatican says.

Others also have noted that the new Pope has yet to formally “take possession” of the basilica of St. John Lateran — also known as the “Mother of All Churches” — and so possibly decided to revert to his previous practice as archbishop.

But the Vatican dismisses this claim.

“Even if he were washing the feet in the Lateran [basilica], I don’t think he’d be choosing 12 priests. He’d be choosing 12 sick people, or 12 homeless people to have their feet washed,” said a Vatican spokesman. “This is a convenient way of doing it, but I don’t know to what degree not taking possession of St. John Lateran has to do with this. He could have done it in St. Peter’s basilica.”

Celebrating the mass with the Pope will be Cardinal Agostino Vallini, vicar general of the Diocese of Rome, and the Rev. Gaetano Greco, chaplain of the Institute. After the mass, the Pope will meet with the youth and the prison staff in the prison gym. Around 150 people, including Italian government officials, are expected to attend.

The Vatican says the youth will give the Pope a wooden crucifix and kneeler which they made themselves in the prison workshop. The Pope is to bring Easter eggs and “colomba” (the traditional Italian Easter cake in the shape of a dove) for detainees and staff.

Given the intimate nature of the pastoral visit, media coverage is restricted and there will be no live broadcasts, the Vatican said.

The Pope’s decision is the latest in a number of breaks with tradition. He chose not to wear the usual papal vestments on the night of his election, traveled by bus with other cardinals, and has worn black instead of traditional red papal shoes.

On Tuesday he said that for now he would rather stay in a Vatican guesthouse rather than move into the palatial papal apartment. The Pope was meant to stay in the Casa Santa Marta guesthouse only on a temporary basis until the apostolic palace renovations were completed.

Each of these moves is consistent with his life as an archbishop, and in line with his personal humility and wish for simplicity, which he sees as a more effective way of evangelization.