The Catholic Church's 115 cardinal electors were on Tuesday taking part in a mass in St. Peter's Basilica ahead of entering the conclave for a papal election that observers say has no clear favourite, dpa reported.
The Pro Eligendo Romano Pontefice ("For the Election of the Roman Pontiff") mass is presided by Angelo Sodano, the elderly dean of the College of Cardinals, and is also open to non-voting cardinals - those aged more than 80.
After the mass, expected to last about two hours, the so-called Princes of the Church are due to head for the Domus Sanctae Marthae (Santa Marta), a modern guesthouse inside the Vatican's grounds, before taking part in a procession into the Sistine Chapel, where they will be locked in from the outside world until a new pontiff is elected.
The first round of voting is due to take place in the late afternoon, with Vatican spokesman Federico Lombardi anticipating black smoke, signalling no pope has yet been elected.
While the selection process is highly secretive, and there are no formal candidates, papal watchers say this election is much more open than the one in 2005 that turned German-born Cardinal Joseph Ratzinger into now Pope Emeritus Benedict XVI after just four rounds of voting.
The shortlist of "papabili" (papal candidates) includes Italy's Angelo Scola, Brazil's Pedro Odilo Scherer, Timothy Dolan of the United States, Marc Ouellet of Canada, Peter Turkson of Ghana and Leonardo Sandri of Argentina.
The race is portrayed as a clash between reformists and "curiali", those loyal to the Roman Curia - the equivalent of the Vatican's government.
"I don't think it (the conclave) will be long," Cardinal Norberto Rivera Carrera told Italian daily La Stampa. "There are differences of opinion, but we will find an agreement very soon."
The next pope will take over a Church beset by infighting, scandal and dwindling support, particularly in the West.