The world watches the Vatican conclave, waiting for the 'white smoke' signaling a new Pope

Some 133 cardinal electors - under the age of 80 - gathered at the Vatican from five continents to choose a successor to Pope Francis, who died last month after 12 years in office.

With experts pointing to leading liberal and conservative candidates from Europe, the US, Asia and Africa, the race to lead the 2,000-year-old institution appears wide open.

At a time of geopolitical uncertainty, the new pope will face complex diplomatic balancing acts, as well as infighting within the Church, the lingering fallout from the child abuse scandal and declining membership in the West.

The "Princes of the Church" (i.e. the Cardinals) will celebrate a pre-conclave Mass in St. Peter's Basilica at 10 a.m. (local time), presided over by the Dean of the College of Cardinals, Cardinal Giovanni Battista Re.

It will be the last public ceremony before the Church's 267th pope is introduced to the world from the balcony of St. Peter's Basilica, hours or possibly days later.

"If we could see the white smoke, it would be something that happens once in a century," American tourist Luke Vanderburgh told AFP on May 7.

Both Pope Francis and his predecessor, Pope Benedict XVI, were elected within two days, but the longest papal election in Church history lasted 1,006 days, from 1268 to 1271.

With clergy from some 70 countries, this conclave is the largest ever and the next pope will have to secure at least 89 votes - a two-thirds majority.

The cardinals are staying at the Vatican's Santa Marta guesthouse - where Pope Francis once lived - and Santa Marta Vecchia, an adjacent building normally reserved for Vatican officials.

At 3:45 p.m. they will depart from Santa Marta to gather at the Pauline Chapel of the Apostolic Palace, where a prayer service will be held from 4:30 p.m.

They then entered the 15th-century Sistine Chapel to begin the conclave, an event "among the most secret and mystical in the world," the Vatican said on May 6.

Oath

Under a ceiling with frescoes by Michelangelo, Italian Cardinal Pietro Parolin - the most senior cardinal elector - will ask God to give the cardinals the necessary "spirit of intelligence, truth and peace".

Cardinal Parolin, a leading candidate who served as Pope Francis' number two as Vatican Secretary of State, will then lead the cardinals in singing the Litany of the Holy Spirit in Latin.

The cardinals spent days discussing the most pressing challenges facing the Catholic Church and the character traits a new leader would need.

Hot issues include the declining number of priests, the role of women, the Vatican's troubled finances and how to adapt the Church to the modern world.

About 80% of the cardinals were appointed by Pope Francis. However, while previous interviews have shown that some cardinals favor a leader who can protect and advance Pope Francis' legacy, others want a more conservative doctrinal defender.

More than a dozen names are being mentioned, from Cardinal Pierbattista Pizzaballa of Italy to Cardinal Peter Erdo of Hungary and Cardinal Malcolm Ranjith of Sri Lanka.

We may never know how close the race was. After handing in their cell phones, the cardinals in red robes will swear an oath to keep the conclave secret.

Each also pledged to "faithfully" serve as pope if elected, before the liturgical master declared "Extra omnes" ("All out").

As the doors closed, the cardinals filled out ballots that read "Eligo in Summum Pontificem" ("I elect... as Supreme Pontiff").

They then folded the ballot, brought it over and placed it on a silver plate that was used to drop it into a ballot urn on a table in front of Michelangelo's "The Last Judgment."

Traditionally, the cardinals cast only one ballot on the first evening, then burn the ballots with a chemical that produces smoke - black smoke means no decision has been made, white smoke signals a new Pope.

Outside, hundreds of faithful are expected to gather in St. Peter's Square, all eyes on the chimney of the Sistine Chapel, with the results expected in the early evening.