VATICAN CITY (Reuters) - Pope Benedict left the Catholic world in shock after becoming the first pontiff since the Middle Ages to resign his office, saying that failing strength had left him unable to lead the church through a period of relentless change and turmoil.
The 85-year-old pontiff announced his abdication as leader of the world's 1.2 billion Catholics in a speech delivered in Latin, the universal language of the church, to cardinals meeting in the Vatican's Apostolic Palace.
"I have come to the certainty that my strengths, due to an advanced age, are no longer suited to the adequate exercise of the Petrine ministry," he said, referring to the tradition that dates the papacy back to Saint Peter, 2,000 years ago.
He will continue in office until 1900 GMT on February 28 before stepping down to allow the election of a new pope, which Vatican officials said was expected to come by the start of the Holy Week on March 24.
He is expected to spend some time at the pope's summer residence near Rome before retiring to spend his final years in a cloistered convent in the Vatican, and will play no part in selecting his successor.
Famously known as "God's Rottweiler" before his election in 2005, Benedict fought against the spread of materialist values in society and strongly opposed any relaxation of the church's traditional strictures against contraception, homosexual acts or women priests.
His eight years in office were overshadowed by scandals ranging from the sexual abuse of children by priests to the arrest of his own butler for stealing confidential papal documents in the so-called "Vatileaks" affair.
The pope said he had left "with full freedom" and Church officials were at pains to stress that the running of the Church would not be affected by his unexpected departure, which surprised even close aides.
While his surprise decision was greeted with respectful tributes from world leaders including U.S President Barack Obama and German Chancellor Angela Merkel, some others underlined the problems which blighted much of Benedict's time in office.
"I deeply respect the decision of Pope Benedict XVI, especially since it is not in line with tradition," said Herman Van Rompuy, president of the European Council and himself a Catholic. "His pontificate has been short but very difficult."
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The complex machinery to choose a successor will move into gear, opening the way for the conclave of cardinals whose decision will be announced with the release of white smoke from a chimney in the Sistine Chapel.
Speculation has grown that the Church could appoint its first non-European leader to reflect the growing weight of regions such as Africa or Latin America, which now accounts for 42 percent of the world's Catholics.
"It could be time for a black pope, or a yellow one, or a red one, or a Latin American," said Guatemala's Archbishop Oscar Julio Vian Morales after Benedict's announcement.
After Benedict's relatively brief papacy, which followed the 27-year pontificate of John Paul II, the cardinals may also be inclined to choose a younger man than Benedict, who was 78 when he was elected.
Whoever is appointed will have to deal with regional issues and the tension between conservative Catholics who have supported Benedict's strictly traditional doctrinal line and others who feel he has stifled change and development.
"In Europe, the Church is seeking a new relationship to society. In many countries in Asia and Africa, it is experiencing an incredible expansion," Archbishop of Vienna Cardinal Christoph Schoenborn said.
Benedict himself had a mixed record in dealing with cultures outside his own, triggering fury among Muslims with a speech critical of Islam in 2006 and angering many in Africa by opposing the use of condoms to combat the scourge of AIDS.
Never as popular as the widely beloved John Paul, Benedict was a scholarly theologian with little of the shrewd political instinct which elevated his predecessor to the front rank of world statesmen.
His decision to leave office shocked some Catholics, who felt that a pope should stay in office until the end of his life, and his exit will leave the Church with both a retired and a serving pope for the first time in hundreds of years.
The last pope to leave office willingly was Celestine V, a saintly hermit who served only a few months before abdicating in December, 1294. Another pope, Gregory XII reluctantly abdicated in 1415 to end a dispute with a rival claimant to the papacy. (Editing by Peter Graff)