The 115 Catholic cardinals tasked with choosing a successor to Pope Benedict XVI celebrated the last Mass in St. Peter's Basilica before retreating from public view.

Wearing the traditional bright red vestments, the cardinals listened to the homily of Cardinal Angelo Sodano, who addressed them for the last time before the start of the conclave, which is held behind closed doors in the Sistine Chapel.

"Each of us is therefore called to cooperate with the Successor of Peter, the visible foundation of such an ecclesial unity," Sodano said.

The mass was broadcast live on giant TV screens in St. Peter's Square. However bad weather conditions kept away many of the worshippers who had arrived in Rome to watch the historic election.

"It's a moment of crisis for the church so we have to show support of the new pope," said Veronica Herrera, a real estate agent from Mexico who travelled to Rome for the conclave with her husband and daughter.

At 3.30pm the cardinals will gather beneath Michelangelo's famous frescoed ceiling and, after having chanted the monophonic Litany of Saints, they will place their hand on the Gospel and swear to observe absolute secrecy before and after the conclave.

Then the papal master of ceremonies, Monsignor Guido Marini, will deliver an extra omnes order ("Everybody out!"), intimating that anybody not linked to the election must leave immediately, and the voting process will begin.

The first meeting is set to end at 7 pm. If by then two-thirds of the cardinals have agreed on the name of the new pontiff, white smoke will raise from the Holy See.

Otherwise a plume of black smoke will emerge from the chapel chimney and the cardinals will be taken to the Vatican hotel, where a ban on use of phones and internet and even on the reading of newspapers is in place to prevent interference with the election.

Cardinals Angelo Scola of Italy and Odilo Scherer from Brazil have emerged as favourites to take the place left vacant by Benedict's surprise resignation, but the competition within the conclave is extraordinarily open.

"I don't think it's going to be a European pope," said Michael Flueckiger, a 38-year-old caretaker and sacristan of a church in Slamatt, Switzerland, who attended the morning mass in St. Peter's Square.

"In Europe sometimes I think we have given away the gift of faith, many people have lost the faith, they have lost their expectation in God."