Pope Benedict XVI announced on Monday he will resign because of old age, becoming the first pontiff in more than six centuries to step down in a move that stunned the world.
The German-born leader of the world's 1.1 billion Catholics said he would resign on February 28 after just eight years as pope, one of the shortest pontificates in modern history.
"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, who succeeded Pope John Paul II in 2005 and is known as a diehard traditionalist and a lightning rod for controversy, will retire to a monastery within the Vatican walls.
He said his "strength of mind and body... has deteriorated in me to the extent that I have had to recognise my incapacity to adequately fulfil the ministry entrusted to me."
He said he would be stepping down at 8 pm (1900 GMT) on February 28, adding that he was "well aware of the seriousness of this act".
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.
Tributes poured in from around the world including his native Germany where Chancellor Angela Merkel hailed him as "one of the most significant religious thinkers of our time".
Justin Welby, leader of the world's Anglicans, said he understood "with a heavy heart" Benedict's decision, and that he had held his office with "great dignity, insight and courage".
The Ashkenazi Chief Rabbi of Israel said Benedict XVI had improved ties between the two religions.
"During his term, the relations between the Chief Rabbinate and the Church, and Judaism and Christianity, became much closer, which brought to a decrease in anti-Semitic acts around the world," a spokesman for Rabbi Yona Metzger told AFP, expressing hope that his successor would continue in the same vein.
Among those tipped by bookmakers to take over are Francis Arinze of Nigeria, Peter Turkson of Ghana and Marc Ouellet of Canada.
Benedict, formerly Joseph Ratzinger, was the Catholic Church's doctrinal enforcer for many years and earned the nickname "God's Rottweiler". He is an academic theologian who has written numerous books including a trilogy on the life of Jesus Christ that he completed last Christmas.
He was elected in 2005 at a time when the Vatican was being rocked by multiple scandals over child abuse committed by priests.
The US Survivors Network of those Abused by Priests, which has been highly critical of Benedict's handling of the paedophilia scandal, called on the Church to "select a Pontiff who puts child safety and victim healing first."
The guiding principle of Benedict's 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. But he has ruffled feathers among Muslims, Jews, gays, AIDs activists and even scientists.
Shocked Catholic faithful began gathering in St. Peter's Square after the news broke, with many saying they were torn over whether Benedict was doing the right thing.
"It's a responsible gesture, it comes from the head, but not from the heart," said David, a 24-year old tourist from Belgium, while Marta, 38, said: "He should have stayed for life."
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", adding that most of the cardinals present for his announcement had not been informed beforehand.
"This gesture was very courageous and revolutionary," said Vatican expert Marco Politi, author of a best-selling biography of Benedict.
"This is the first time that in a period of peace for the Church, a pope decides to step down of his own free will," he said.
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 saying he yearned for a simpler life and was not physically capable for the office.
In 1415, Gregory XII was forced to leave to end the "Western Schism", when two rival claimants declared themselves pope in Pisa and Avignon and threatened to tear apart Roman Catholicism.