Cardinals from across the world will be shut into the Sistine Chapel on Wednesday for a secretive conclave to elect the next head of the world's 1.4 billion Catholics.
Some 133 cardinal electors -- aged under 80 -- have gathered at the Vatican from five continents to pick a successor to Pope Francis, who died last month after a 12-year-long papacy.
With experts singling out liberal and conservative frontrunners from Europe, the United States, Asia and Africa, the race to lead the 2,000-year-old institution appears wide open.
In a time of geopolitical uncertainty, the new pope faces diplomatic balancing acts, as well as Church infighting, the continued fall-out from the clerical child abuse scandal, and increasingly empty pews in the West.
The "Princes of the Church" will hold a pre-conclave mass in St Peter's Basilica at 10:00 am (0800 GMT), presided over by the dean of the College of Cardinals, Giovanni Battista Re.
It will be the last rite to be celebrated publicly before the Church's 267th pope is presented to the world from a balcony of St Peter's Basilica, several hours or perhaps days later.
"If we can witness the white smoke that'd be something... that's definitely once in a lifetime," US tourist Luke Vanderburgh told AFP on Tuesday.
Both Francis and his predecessor Benedict XVI were elected within two days, but the longest papal election in Church history lasted 1,006 days, from 1268 to 1271.
With clerics from around 70 countries, this conclave is the largest ever and the next pontiff will have to secure at least 89 votes -- a two-thirds majority.
The cardinals are staying at the Vatican's Santa Marta guesthouse -- where Francis used to live -- and Santa Marta Vecchia, a building next door usually housing Vatican officials.
At 3:45 pm they will set off from Santa Marta to gather at the Pauline Chapel of the Apostolic Palace, where a prayer will be held from 4:30 pm.
They then proceed into the 15th-century Sistine Chapel for the conclave, which is "one of the most secret and mysterious events in the world", the Vatican said on Tuesday.
Under a ceiling of frescoes painted by Michelangelo, Italian Cardinal Pietro Parolin -- the senior elector -- will call on God to give the cardinals "the spirit of intelligence, truth and peace" needed.
Parolin, a frontrunner who was Francis's number two as secretary of state, will then lead the cardinals in chanting the Latin invocation of the Holy Spirit: "Veni, Creator Spiritus".
The cardinals have spent days discussing the most pressing challenges facing the Catholic Church and the character traits its new leader needs.
Burning issues include falling priest numbers, the role of women, the Vatican's troubled balance sheets and how to adapt the Church to the modern world.
Some 80 percent of the cardinals were appointed by Francis -- an impulsive, charismatic defender of the weak.
But while interviews in the run-up suggested that some cardinals favour a leader able to protect and develop his legacy, others want a more conservative defender of doctrine.
More than a dozen names are circulating, from Italian Pierbattista Pizzaballa to Hungary's Peter Erdo and Sri Lanka's Malcolm Ranjith.
We may never know how close a race it is. Having surrendered mobile phones, the red-robed cardinals will swear an oath to keep the conclave's secrets.
They also each pledge to "faithfully" serve as pope should they be chosen, before the master of liturgical ceremonies says "Extra omnes" ("Everyone out").
Once the doors close, the cardinals fill out ballots marked "Eligo in Summum Pontificem" ("I elect as Supreme Pontiff").
They then carry them, folded, and place them on a silver plate which is used to tip them into an urn, set on a table in front of Michelangelo's Last Judgment.
The cardinals traditionally cast just one ballot on the first evening, burning the votes along with a chemical which will produce smoke -- black for no decision, white for a new pope.
Outside, hundreds of the faithful are expected to gather on St Peter's Square, all eyes trained on the Sistine Chapel chimney, with a result expected by early evening.
Bd-Pratidin English/ARK
