The Mass in St. Peter's Basilica was led by 78-year-old German Cardinal Joseph Ratzinger, the Vatican's doctrinal chief who is seen by some as a leading candidate for the papal throne.

"O God, eternal pastor who governs your people with the care of a father, grant your Church a pontiff acceptable to you for his holy life and entirely consecrated to the service of your people," Ratzinger said in the opening prayer.

All of the 115 cardinal electors under the age of 80 who will enter the conclave joined older cardinals, bishops, priests, nuns and simple faithful in united prayer for the outcome of the conclave.

The conclave, a unique election mixing ancient ritual with ultra-modern technology was due to start at about 4:30pm (10:3 0am EDT).

In a process dating back to medieval times, the "Princes of the Church" will shut themselves in the Sistine Chapel and other restricted parts of the Vatican, emerging only when they have chosen the first new pontiff of the third Christian millennium and the 264th successor to St. Peter.

If the conclave resembles previous ones, the cardinals will need several days and repeated votes to reach a majority.

Whoever they choose will have to try to make his mark in the shadow of one of the most dynamic papacies in history.

"This is a very difficult papacy to inherit," said Orietta Cristoferi outside St. Peter's Basilica. "He should have an openness to Latin America and other places. John Paul worked hard to open up to the whole world."

The conclave appears wide open with no sign of who will emerge as leader of the world's 1.1 billion Roman Catholics.