Pope Francis, starting four packed days of events culminating in Easter, said on Thursday the church must always be a refuge for the poor, the homeless and the sick.
Francis, who since his election last year has urged priests to shun material comforts or the urge to climb clerical career ladders, led a solemn Holy Thursday service on the day that Christians commemorate the founding of the priesthood by Jesus.
In the grandeur of St Peter's Basilica, he celebrated a "Mass of the Chrism" during which he and priests renewed the vows they took on the day of their ordination and he blessed oils to be used in sacraments during the year.
At the mass, attended by some 10,000 people and accompanied by a pontifical choir, he delivered a sermon on the need for priests to live a simple, poor life.
He described the church as "a house with open doors, a refuge for sinners, a home for people living on the streets, a place of loving care for the sick, a camp for the young, a classroom."
Holy Thursday commemorates the day on which Catholics believe Jesus founded the priesthood, at the Last Supper with his apostles before he was betrayed and arrested on the night before his crucifixion.
Later on Thursday, Francis was to travel to a rehabilitation centre on Rome's outskirts for a service in which he was to wash and kiss the feet of 12 sick and disabled people, in commemoration of Jesus' gesture of humility towards his apostles on the night before he died.
The 12 range in age from 16 to 86 and include four women and a Muslim man.
Previous popes always held the ceremony in the Vatican or Rome's St. John's Basilica and only included 12 Catholic men, usually priests, at the service.
But Francis, continuing a tradition he started when he was archbishop of Buenos Aires, holds it in peripheral neighbourhoods with ordinary people.
Last year he held it in a juvenile jail and washed and kissed the feet of women and Muslim inmates.
On Good Friday and Holy Saturday Francis presides at three services leading up to Easter Sunday, when he delivers his "Urbi et Orbi" (to the city and the world) blessing and message.
On Sunday, April 27, Francis canonises Pope John Paul II, who reigned from 1978 to 2005, and Pope John XXIII, who was pontiff from 1958 to 1963 and called the Second Vatican Council, a landmark meeting that modernised the Church.