Religious and Social Affairs Correspondent Joe Little reports on the start of the conclave, as cardinals enter the chapel in procession and swear secrecy about the proceedings.
Cardinals have processed into the Sistine Chapel in the Vatican, where they will elect a successor to Pope Benedict XVI.
Chanting the Litany of the Saints, the 115 most senior prelates in the Catholic Church walked in double file the short distance from the Pauline Chapel to the Sistine Chapel.
There they bowed before the altar before taking their places standing behind tables, where they will write their selection onto ballot papers that contain no pre-printed candidates' names.
Vested in their trademark scarlet robes, they continued singing "Come, O Creator Spirit blessed. And in our souls take up Thy rest."
Leading the procession was the Dean of the Electoral College, Cardinal Giovanni Battista.
Next came four most senior members of the College of Cardinals, called the cardinal bishops.
They were followed by 81 cardinal priests, including Archbishop Seán Brady who, in turn, were followed by 30 cardinal deacons.
Cardinal Brady was three positions behind Cardinal Seán Patrick O'Malley of Boston as he approached the bible on which each elector places his hand while swearing the oath.
Dr Brady, who is 73, is the first Archbishop of Armagh to vote in a conclave since the late Cardinal William Conway in 1957.
Cardinal Desmond Connell voted in the 2005 conclave, which elected Pope Benedict XVI, who sensationally resigned last month in the first move of its kind in over seven centuries.
Dr Connell is ineligible to vote this time because he is over the cut-off age of 80.
Last in the procession were officials such as the pontifical Master of Ceremonies and the Secretary to the conclave.
They assisted in the administration of an oath to maintain the strictest secrecy about all that occurs during the open-ended electoral process.
After each of the 115 electors had sworn the oath, the Master of Pontifical Liturgical Celebrations gave the order "Extra Omnes", meaning "All Out", and all unauthorised persons had to leave.
But he himself remained behind along with Cardinal Grech of Malta, who was chosen to address the electors on the gravity of the task before them and the paramount need for integrity of intention and wise discernment in choosing a new pope, with eyes fixed in God alone.