Pope Benedict XVI has announced he will resign this month because of his advancing age in a fast-changing world, becoming the first pontiff in 700 years to step down of his own free will.
The 85-year-old said he will step down on February 28 after just eight years in office, making his one of the shortest pontificates in modern history and stunning the world's 1.1 billion Catholics.
The German-born leader made the announcement Monday in a speech in Latin during a meeting with cardinals at Apostolic Palace residence. His frail voice was barely audible as he read the text.
"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," he said.
As tributes poured in from across the world, the Vatican emphasised that the former Joseph Ratzinger was not leaving due to any illness, despite speculation over his increasingly frail appearance.
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.
"The pope caught us a bit by surprise," Lombardi admitted at a hastily arranged press conference.
For Cardinal Angelo Sodano, who attended the historic meeting with the pope, the announcement came as "a bolt of lightning in a clear blue sky."
Benedict's brother Georg Ratzinger told AFP he had known for "a few months" that the pope was planning to resign and was "feeling the burden of his age."
Some faithful said they hoped the move would signal a major change for the Church after a conservative pontificate marred by scandals, most notably over clerical child sex abuse.
Vatican observers have already begun speculating over who could succeed Benedict.
Online betters tip an African pope as the most likely, but other observers say the number of voting-age cardinals from Europe and North America -- 76 out of 118 -- could sway the choice to a Western state.
In the period between the resignation and the election of a new pope, the Catholic Church will be governed by Secretary of State Tarcisio Bertone.
The closed-door conclave of cardinals is held in the Sistine Chapel, a puff of white smoke indicating when they have decided on a successor.
The new pope is then announced immediately afterwards with the cry "Habemus Papam" and appears before the faithful gathered in St Peter's Square.
German Chancellor Angela Merkel led tributes from political and religious leaders across the globe, hailing Benedict as "one of the most significant religious thinkers of our time."
US President Barack Obama offered "our appreciation and prayers" on behalf of all Americans. A spokesman for Ban Ki-moon said the UN chief had praised the pope's commitment to interfaith dialogue.
Justin Welby, spiritual leader of the world's Anglicans, said the pope 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, helping reduce anti-Semitism around the world.
Shocked believers flocked to St Peter's Square to express their dismay at what will be only the second formal resignation in the Catholic Church's 2,000-year history after Celestine V in 1294.
"I love Benedict. We're really shocked he's resigning because he wasn't pope for long enough. He hasn't finished his plan," said Sebastian Mazur, a 21-year-old trainee priest from Poland.
Gian Maria Vian, editor of the Vatican's official daily L'Osservatore Romano, said the pope took his decision after a particularly wearying trip to Mexico and Cuba last year and only following "a repeated examination of his conscience."
Benedict, who succeeded the late pope John Paul II in 2005, is known as a diehard traditionalist and a lightning rod for controversy. He will retire to a monastery within the Vatican walls.
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," the pope said.
He plans to step down at 8:00 pm (1900 GMT) on February 28.
Benedict, who has often had to use a mobile platform to move around St Peter's Basilica during church services, had hinted in 2010 at a possible resignation if he felt he was no longer able to carry out his duties.
The pope suffers from arthritis, had a stroke while he was still cardinal and broke a wrist when he slipped in the bath in 2009.
He was the Catholic Church's doctrinal enforcer for many years, earning the nickname "God's Rottweiler." An academic theologian, the pope has written numerous books, including a just completed trilogy on the life of Jesus Christ.
The guiding principle of Benedict's papacy has been to reinvigorate the Catholic faith, particularly among young people and in parts of the world with rising levels of secularism, like Europe and North America.
He has shown a degree of openness on some moral issues, becoming the first pope ever to speak about the possibility of using contraception to avoid the spread of the AIDS virus.
But he is better known for his opposition to abortion, euthanasia and gay marriage -- as well as the abuse scandals that tainted the Church.
The US Survivors Network of those Abused by Priests, 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."
Sandro Magister, a Vatican expert at L'Espresso weekly, said the resignation could set a precedent.
"He must have evaluated the effects of the resignation on future pontificates, which will definitely not be for life but will become fixed-term," Magister said.
Marco Politi, author of a best-selling biography of Benedict, said the gesture was "very courageous and revolutionary."
"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 added.