VATICAN CITY: Cardinals entered the Sistine Chapel in a procession on Tuesday for an historic conclave to elect a new pope after Benedict XVI's shock resignation, with the odds favouring a Western conservative to lead the Catholic world.

The 115 scarlet-robed cardinals who will choose the next leader of the world's 1.2 billion Catholics filed in past Swiss Guards under Michelangelo's famous frescoes of the Last Judgment and God's hand outstretched to Adam.

They sang the Latin hymn "Veni Creator Spiritus" ("Come Creator Spirit"), asking for divine guidance at a time in which the Church is beset by troubles -- from increasing secularism in the West and the rise of radical Islam to the ongoing scandal over child abuses by priests.

In a chapel swept for bugs and with jamming devices in place to prevent electronic communications, each cardinal swore a solemn oath with their hand on the Bible not to reveal the secrets of their deliberations or face being cast out from the Church.

At a grandiose 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 eight-year pontificate.

As rainstorms drenched thousands of pilgrims in St Peter's Square, the "Princes of the Church" 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.

The cardinals will be entirely cut off from the outside world inside the Vatican walls until they have made their choice in a series of elaborate rituals dating back to the 13th century.

Several 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 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 famously turns 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 II's death took just two days.

Among the candidates, three have emerged as favourites -- Italy's Angelo Scola, Brazil's Odilo Scherer and Canada's Marc Ouellet -- all of them conservatives like "pope emeritus" Benedict XVI.

But cardinals from Austria, Hungary, Mexico, the Philippines, South Africa and the United States 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.

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."