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Pope Benedict XVI's final Sunday prayers in St Peter's Square will signal the start of a week in the Vatican that will make history with the first voluntary papal resignation in more than 700 years.
Tens of thousands of pilgrims have gathered in Rome from around the world following the ageing pope's shock announcement that he would step down because he no longer had the strength of body and mind to govern the Catholic Church in modern times.
Crowds are expected to turn out in the Vatican on Sunday and again on Wednesday when the leader of the world's 1.2 billion Catholics will bid a final farewell to ordinary Catholics at a general audience in the Vatican's 17th-century plaza.
On Thursday, the pope will meet his cardinals in the morning and take a helicopter to the papal summer residence of Castel Gandolfo in the afternoon.
At 1900 GMT that same day, Benedict will become only the second pope in the Church's 2,000-year history to resign of his own free will.
Speculation has been rife in recent days over what might have triggered the pope's decision, with the Panorama weekly and the Repubblica daily saying it was linked to an explosive report with allegations of blackmail against gay Vatican clergymen.
The Vatican has refused to comment on rumours about the secret report compiled by a committee of cardinals investigating leaks of hundreds of confidential documents from the papal office.
The cardinals uncovered intrigue, factionalism and corruption in the Roman Curia, the reports said.
Other recent reports say the resignation was due to the pope's health condition, which they say may be more serious than the Vatican has revealed.
Benedict has said he will live "hidden from the world" -- first at Castel Gandolfo and then in April or May, once renovations are complete, in a former monastery within the Vatican walls on a hillside overlooking St Peter's Basilica.
The Vatican has said the pope's divine infallibility -- the power of making unquestionable utterances on matters of doctrine -- will abandon him the moment he is no longer the pontiff.
The resignation is a revolutionary act that Vatican experts say could set a precedent for future popes when their energies inevitably begin to flag.
It has also caused dismay and in some cases disappointment among many ordinary Catholics who expect popes to serve until their natural deaths.
The only other pope to resign in similar circumstances was Celestine V, a hermit who stepped down just months after being elected in 1294 because he said he was not physically able and was disgusted with the intrigue of Rome.
-- Uncharted waters for the Church --
Quite unprecedented is the idea of a pope and his predecessor living within a stone's throw of each other inside the Vatican -- uncharted waters for the Church which will require sensitive handling.
The surprise has been so great that the Vatican has not determined what titles Joseph Ratzinger will have -- "Bishop Emeritus of Rome" has been mooted -- and whether he can continue to wear papal whites.
The pope will likely continue his theological research but the Vatican has said he could also provide "spiritual guidance" for his successor.
The Vatican's culture minister, Gianfranco Ravasi, even likened the idea of the former pope in his hilltop monastery to the Biblical figure of Moses interceding with God on a mountain as battles raged in the valley below.
Immediately after the resignation, cardinals will hold conferences known as "congregations" starting on Friday that are a way of identifying priorities for the Church and sussing out viable candidates for the papacy.
In the interim before the election of a new pope, the Church will be run by Cardinal Tarcisio Bertone -- a divisive figure who has been Benedict's right hand man for years.
Bertone will have the responsibility of disposing of the "Fisherman's Ring" -- a personalised gold signet ring that is a papal symbol and was traditionally destroyed after a pope's death to prevent his seal being put on fake documents.
Bertone and the cardinals will then set a date for the start of a conclave to which the world's 117 "cardinal electors" -- cardinals below the age limit of 80 -- are all invited to elect a new pope.
Indonesian cardinal Julius Riyadi Darmaatmadja -- one of the 117 -- has already said he will not be travelling to Rome because of his ailing health.
There are doubts too over the presence of US cardinal Roger Mahony, who is accused of covering up sexual abuses committed by paedophile priests in his archdiocese of Los Angeles for many years.
No clear favourite has emerged although the pope's announcement indicates the need for a younger pope.
Vatican watchers say cardinals may also opt for a more pastoral, less academic figure.
Under usual rules, the conclave has to start between 15 and 20 days after the pope passes away but Vatican lawyers are hard at work to see if that timeline can be brought forward in this case.
The cardinals meet behind closed doors under Michelangelo's frescoes in the Sistine Chapel and their deliberations are carried out under a strict vow of secrecy on pain of excommunication.
A conclave can last for days before any candidate wins a two-thirds majority, after which the nominee is asked if he accepts and the ballots are burnt in a special stove that puts out white smoke.
The new pope then retreats to a "Room of Tears" where he dons the papal vestments and emerges onto a balcony over St Peter's Square to the Latin cry of "Habemus Papam!" ("We have a pope!")