Pope Francis has washed the feet of 12 disabled and elderly people, including women and non-Catholics, in a pre-Easter ritual designed to show his willingness to serve others.

His decision in 2013 to perform the Holy Thursday ritual on women and Muslim inmates at a juvenile detention centre helped to define his rule-breaking papacy just two weeks after his election. It riled traditionalist Catholics, who pointed to the Vatican's own regulations that the ritual be performed only on men since Jesus's 12 apostles were men.

But as archbishop of Buenos Aires, Cardinal Jorge Mario Bergoglio frequently performed the ritual on women - a practice that he seems intent on keeping up now that he is Pope.

Holy Week 2014 took Pope Francis to a centre for the disabled and elderly in Rome. He knelt down, washed, dried and kissed the feet of a dozen people, some in wheelchairs, some with grossly swollen and disfigured feet.

The Vatican did not release the exact breakdown of their religious backgrounds, but said they came from various religious confessions. Italian news reports said one was a Libyan Muslim and four were women.

Pope Francis told the faithful that he was performing the ritual to remind himself how to serve others, as Jesus did when he washed the feet of his apostles.

"Jesus made a gesture, a job, the service of a slave, a servant," he said. "And he leaves this inheritance to us: We need to be servants to one another."

Pope Francis began Holy Thursday by presiding over Mass in St Peter's Basilica, celebrating the priesthood, the start of a busy four days of Holy Week commemorations and preparations for next week's canonisation of Popes John Paul II and John XXIII.

During his morning homily, Pope Francis urged his priests to exhibit joy, though he admitted that he too had suffered "moments of listlessness and boredom which at times overcome us in our priestly life".