Pope Benedict XVI on Monday announced he would resign, citing old age, in a stunning announcement that marked a first in the modern history of the Catholic Church.
The German-born pope said he would step down on February 28, which will make him the first pontiff to resign in centuries.
"I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry," the 85-year-old pope said in a speech delivered in Latin at a meeting of cardinals in the Vatican.
Dressed in red vestments and his voice barely audible as he read from a written text, the pope made the announcement in a hall in his residence -- the Apostolic Palace next to St Peter's Square.
Vatican spokesman Federico Lombardi said he expected a conclave of cardinals to be held in March within 15 or 20 days of the resignation and a new pope elected before Easter Sunday on March 31.
Benedict, an academic theologian who has written numerous books including a trilogy on the life of Jesus Christ that he completed last Christmas, will retire to a monastery within the Vatican walls.
"In order to govern the ship of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognise my incapacity to adequately fulfil the ministry entrusted to me," the pope said.
"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter, entrusted to me by the Cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours (1900 GMT), the See of Rome, the See of Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked," he said.
Tributes poured in for Benedict from around the world including his native Germany where Chancellor Angela Merkel said she had the "greatest respect" for his decision, and hailed him as "one of the most significant religious thinkers of our time".
French President Francois Hollande said the pope's decision was "eminently respectable".
British Prime Minister David Cameron said the pope had worked "tirelessly" to boost ties with Britain.
Benedict, formerly Joseph Ratzinger, was the Catholic Church's doctrinal enforcer for many years and earned the nickname "God's Rottweiler".
He was elected in 2005 at a time when the Vatican was being rocked by multiple scandals over child abuse committed by priests.
The guiding principle of his papacy has been to reinvigorate the Catholic faith, particularly among young people and in countries withing rising levels of secularism like Europe and North America.
Benedict has championed Christianity's European roots and showed his conservatism by repeatedly stressing family values and fiercely opposing abortion, euthanasia and gay marriage.
Pope Benedict, who has looked increasingly weary in recent months and often has to use a mobile platform to move around St Peter's basilica during Church services, had hinted in a book of interviews in 2010 that he might resign if he felt he was no longer able to carry out his duties.
The scandal over confidential memos leaked from the Vatican by the pope's once loyal butler last year was a particularly hard blow for the pope.
"The pope caught us a bit by surprise," Vatican spokesman Federico Lombardi said at a hastily-arranged press conference.
Lombardi stressed that the pope's decision was his own and was "well thought out" and that "there is no illness that has contributed to it".
He said the pope had chosen a consistory to make his announcement because it would group cardinals together, adding that most of them had not been informed of the pope's announcement beforehand.
The only other pope to resign because he felt unable to fulfil his duties was Celestine V in 1294, a hermit who stepped down after just a few months in office saying he yearned for a simpler life and was not physically capable for the office.
In 1415, Gregory XII resigned in a bid to end the "Western Schism", when two rival claimants declared themselves pope in Pisa and Avignon and threatened to tear apart Roman Catholicism.
Other popes have stepped down for a variety of reasons in the papacy's mediaeval history.