Cardinals began a conclave under Michelangelo's famous frescoes in the Sistine Chapel on Tuesday to elect a successor to Benedict XVI, with the odds favouring another Western conservative as pope.
The 115 scarlet-robed cardinals filed into the chapel past liveried Swiss Guards, chanting the Latin hymn "Veni Creator Spiritus" ("Come Creator Spirit") to ask for divine guidance after Benedict's troubled eight-year papacy.
The "Princes of the Church" swore on the Bible never to reveal the secrets of their deliberations or face being cast out of the Church forever, before the doors of the chapel swung shut to indicate the beginning of the lock-in.
The chapel has been swept for bugs and installed with jamming devices to prevent electronic communications, with cardinals entirely cut off from the outside world behind the Vatican's walls until they have elected a new pontiff.
At an extravagant pre-conclave mass in St Peter's Basilica the cardinals prayed for unity in the Church -- a stark reminder of the infighting that often overshadowed Benedict's reign over the world's 1.2 billion Catholics.
As rainstorms drenched thousands of pilgrims in St Peter's Square, the cardinals burst into thunderous applause when the dean of the College of Cardinals, Angelo Sodano, thanked the "beloved and venerable" Benedict in his homily.
On the Vatican's 17th-century plaza, a barefoot pilgrim in a sackcloth habit knelt on the cobblestones in the pouring rain and bowed his head in prayer as the chants from the mass echoed across the square from four giant screens.
Several cardinals took to Twitter to say their goodbyes to their online flock before the conclave.
"Last tweet before conclave: May Our Father hear and answer with love and mercy all prayers and sacrifices offered for fruitful outcome. God bless!" South African Cardinal Wilfrid Napier said.
The cardinals -- who were all appointed by Benedict XVI or his predecessor John Paul II -- were set to hold a first round of voting later on Tuesday after which the ballots will be burnt in a special stove in the Sistine Chapel.
The smoke is famously turned white if there is a new pope but the Vatican has already said it expects the smoke from the burning of Tuesday's ballots to be black, indicating no pope has been elected.
From Wednesday, ballots will be burnt after two rounds of voting in the morning and two rounds in the afternoon -- with the eyes of the world focused on the colour of the wisps of smoke emanating from a special chimney installed on the chapel's roof.
Modern-day conclaves normally last no more than a few days and a two-thirds majority is required.
The longest conclave in the past century -- in 1922 -- lasted five days. Benedict's election in 2005 following John Paul's death took just two days.
-- Trio of frontrunners --
Among the candidates, three have emerged as favourites -- Italy's Angelo Scola, Brazil's Odilo Scherer and Canada's Marc Ouellet.
Like "pope emeritus" Benedict XVI, all of them combine conservative views with social advocacy on issues like poverty, conflict and immigration.
But cardinals from Austria, Hungary, Mexico, the Philippines, South Africa and the United States -- some of them with more reformist outlooks -- were also rumoured to be in contention.
Bookmakers said Scola was the favourite, followed by Ghana's Peter Turkson and Scherer.
Ireland's Paddy Power and Britain's William Hill said Scola's chances had improved dramatically and both gave the Milan archbishop odds of 9/4.
But Mexican Cardinal Norberto Rivera Carrera told Italian daily La Stampa there was no agreement yet among the cardinals.
"Some imagine him to be more academic, able to establish a dialogue with culture. Others ask for someone who is close to the people. Others still want someone with more authority to put some Church problems in order," he said.
"So far, there is no majority."
A few key aims unite many of the cardinals, like reforming the intrigue-ridden Vatican bureaucracy and re-igniting Catholic faith in the way the charismatic John Paul II did.
There have been calls too from within the Church for a rethink of some basic tenets such as priestly celibacy, the uniform ban on artificial contraception and even allowing women to be priests as in other Christian denominations.
The tradition of holding conclaves goes back to 1268 when cardinals were locked into the papal palace in Viterbo near Rome by an angry crowd because they were taking too long to choose a pope.
Their conclave still dragged on for three years.
Italian media reported the cardinals will be served plain but wholesome food at their Vatican residence -- where the windows are locked shut and telephones are for internal use only.
Hungry prelates will be served "meals of soup, spaghetti, small meat kebabs and boiled vegetables", the Corriere della Sera reported.
"Perhaps this food, similar to that served in hospitals, will help to speed up the choice of a successor," the paper said.
The 85-year-old Benedict announced on February 11 that he no longer had the strength of body and mind to keep up with the modern world.
In a series of emotional farewells, the German said he would live "hidden from the world" and wanted only to be "a simple pilgrim".
Paula Murphy from Ireland, who came with her parish for the conclave, said she was hopeful the election would breathe new life into the Church.
"I'm so excited! It's a moment for renewal."