Catholic cardinals had a final day of jockeying for position on Monday before shutting themselves into the Sistine Chapel to elect a new pope after Benedict XVI's shock resignation, with an Italian and a Brazilian who both head powerful archdioceses among the top contenders.
The cardinals held their last pre-conclave talks where they have been debating the challenges that the next pope will face and vetting possible candidates for the post.
Vatican insiders put Milan Archbishop Angelo Scola in the lead, but without the support of two-thirds of the 115 "cardinal electors" needed to become the new leader of the world's 1.2 billion Catholics.
Brazilian Odilo Scherer, the charismatic archbishop of Sao Paolo and Latin America's best hope, is also seen as in with a chance after the red-frocked cardinals begin the storied process, cloaked in secrecy, of choosing one of their peers to lead the Church.
The electors must take a solemn oath of secrecy or face excommunication -- though no examples of such a fate appear in the record, and Vatican journalists have shown a wily knack for extracting insider information.
Vatican staff members who will work around the conclave from Tuesday were also due to take the same oath later on Monday.
In churches across Rome on Sunday, many of the hopefuls celebrated mass during which they prayed for the divine guidance that is traditionally sought in making their choice.
US Cardinal Sean O'Malley said in his homily that the Catholic world was "united in prayer" as the clock ticked down to the conclave starting Tuesday.
"Let us pray that the Holy Spirit enables the Church to choose a new pope who will confirm us in our faith and make more visible the love of the Good Shepherd," O'Malley told parishioners in Santa Maria della Vittoria church.
The cardinals -- all 115 of whom were appointed by Benedict or his predecessor John Paul II -- will again invoke God's guidance as they process solemnly into the Sistine Chapel, which will be swept for listening devices to keep would-be spies at bay.
Their task is to find a pope -- the 266th -- strong enough to grapple with the challenges assailing the Catholic Church that proved too much for 85-year-old Benedict.
His resignation -- the first for 700 years -- has focused attention on the need to find a leader with the energy to shape the Church's approach to growing secularism in the West and the Islamic radicalism spreading to many parts of the globe.
Cardinals have expressed a desire for a more vigorous, pastoral figure to deal with the relentless scandals over sexual abuse by paedophile priests and cover-ups by superiors that have rocked the Catholic Church.
"Critics would say the most important piece of unfinished business" is cracking down on bishops who protect paedophile priests, said Vatican expert John Allen of the National Catholic Reporter.
"The Vatican now has tough policies against priests who abuse, but it does not have equally tough policies for bishops," Allen told AFP.
The cardinals also want a man who can reform the Roman Curia, the central government of the Catholic Church, which has been beset by the intrigue laid bare in documents leaked by Benedict's butler last year.
While Canadian Marc Ouellet has attracted attention as the powerful prefect of the Congregation of Bishops, Scola was greeted by a mass of photographers and cameramen when he celebrated mass in the Church of the Twelve Holy Apostles on Sunday.
Scola, 71, a hardliner cut from the same cloth as Benedict, the German-born arch-conservative Joseph Ratzinger, has the advantage of not being associated with the tarnished Vatican bureaucracy.
The conclave, named after the Latin phrase meaning "with key", comes eight centuries after the first lock-in, when cardinals who had dithered over their choice for nearly three years were given just bread and water until they finally made up their minds.
Another leading contender, according to Italian media, is American prelate Timothy Dolan -- even though the cardinal told one interviewer that anyone mentioning him as a candidate must be "smoking marijuana".