The 115 cardinals held a first inconclusive vote in the Sistine Chapel yesterday as they began the process of finding a successor to Benedict XVI, who brought a troubled eight-year papacy to an abrupt end by resigning last month.
Black smoke billowed into the night air above the Vatican, indicating that no-one had gained the two-thirds majority needed to become the 266th Roman pope.
White smoke -- produced by mixing the smoke from burning ballots with special flares -- would indicate that a new head of the Roman Catholic Church has been chosen.
As they awaited the outcome of the first vote, suspense mixed with hopes among the tens of thousands of pilgrims in St Peter's Square -- and in the Catholic Church worldwide, which is struggling in many parts with scandals, indifference and conflict.
Among the cardinals Italy's Angelo Scola, Brazil's Odilo Scherer and Canada's Marc Ouellet -- all conservatives like Benedict -- are the three favourites but there is no clear frontrunner and conclaves are notoriously difficult to predict.
Some analysts suggest that Benedict's dramatic act -- the first papal resignation in over 700 years -- could push the cardinals to take an equally unusual decision and that an outsider could emerge as a compromise candidate.
Hopes are high in the Philippines for the popular Archbishop of Manila, Luis Antonio Tagle, and on the African continent for South Africa's Wilfrid Napier, the archbishop of Durban, but in practice their chances are very slim.
Two-thirds of the cardinals are from Europe and North America and the view among many experts is that only someone with experience of its inner workings can reform the scandal-tainted Vatican bureaucracy, the Roman Curia.
The cardinals yesterday filed into the chapel, chanting a Latin hymn to ask for divine guidance and swearing a solemn oath never to reveal the secrets of their deliberations on pain of excommunication.
The "Princes of the Church" are cut off from any contact with the outside world for the duration of the conclave. They eat and sleep in a Vatican residence where windows are locked shut and phones are for internal use only. Modern-day conclaves normally last no more than a few days.
Benedict's election in 2005 following the death of John Paul II took just two days.
Dressed in their scarlet robes, traditionally symbolising the blood they are willing to spill in the service of the Church, the cardinals held a pre-conclave mass in St Peter's Basilica where they prayed for unity.