VATICAN CITY -- With a few words in Latin, Pope Benedict XVI did what no pope has done in more than half a millennium, stunning the world by announcing his resignation Monday and leaving the already troubled Catholic Church to replace the leader of its 1 billion followers by Easter.

Not even his closest associates had advance word of the news, a bombshell that he dropped during a routine meeting of Vatican cardinals. And with no clear favorites to succeed him, another surprise likely awaits when the cardinals elect Benedict's successor next month.

"Without doubt this is a historic moment," said Cardinal Christoph Schoenborn, a protege and former theology student of Benedict's who is considered a papal contender. "Right now, 1.2 billion Catholics the world over are holding their breath."

The Feb. 28 resignation allows for a fast-track conclave to elect a new pope, since the traditional nine days of mourning that would follow a pope's death doesn't have to be observed. It also gives the 85-year-old Benedict great sway over the choice of his successor. Though he will not himself vote, he has hand-picked the bulk of the College of Cardinals -- the princes of the church who will elect his successor -- to guarantee his conservative legacy and ensure an orthodox future for the church.

The resignation may mean that age will become less of a factor when electing a new pope, since candidates may no longer feel compelled to stay for life.

"For the century to come, I think that none of Benedict's successors will feel morally obliged to remain until their death," said Paris Cardinal Andre Vingt-Trois.

Benedict said as recently as 2010 that a pontiff should resign if he got too old or infirm to do the job, but it was a tremendous surprise when he said in Latin that his "strength of mind and body" had diminished and that he couldn't carry on. He said he would resign effective 8 p.m. local time on Feb. 28.

"All the cardinals remained shocked and were looking at each other," said Monsignor Oscar Sanchez of Mexico, who was in the room at the time of the announcement.

As a top aide, Benedict watched from up close as Pope John Paul II suffered publicly from the Parkinson's disease that enfeebled him in the final years of his papacy. Clearly Benedict wanted to avoid the same fate as his advancing age took its toll, though the Vatican insisted the announcement was not prompted by any specific malady.

The Vatican said Benedict would live in a congregation for cloistered nuns inside the Vatican, although he will be free to go in and out. Much of this is unchartered territory. The Vatican's chief spokesman, the Rev. Federico Lombardi, said he isn't even sure of Benedict's title -- perhaps "pope emeritus."

Since becoming pope in 2005, Benedict has charted a very conservative course for the church, trying to reawaken Christianity in Europe where it had fallen by the wayside and return the church to its traditional roots, which he felt had been betrayed by a botched interpretation of the modernizing reforms of the Second Vatican Council.

His efforts though, were overshadowed by a worldwide clerical sex abuse scandal, communication gaffes that outraged Jews and Muslims alike and, more recently, a scandal over leaked documents by his own butler. Many of his stated priorities as pope also fell short: He failed to establish relations with China, heal the schism and reunite with the Orthodox Church, or reconcile with a group of breakaway, traditionalist Catholics.

There are several papal contenders in the wings, but no obvious front-runner -- the same situation as when Benedict was elected after the death of John Paul. As in recent elections, some push is expected for the election of a Third World pope, with several names emerging from Asia, Africa and Latin America, home to about 40 percent of the world's Catholics.

The Vatican stressed that no specific medical condition prompted Benedict's decision, saying he remains fully lucid and took his decision independently.

"Any interference or intervention is alien to his style," Lombardi said.

The pope has clearly slowed down significantly in recent years, cutting back his foreign travel and limiting his audiences. He now goes to and from the altar in St. Peter's Basilica on a moving platform to spare him the long walk down the aisle. Occasionally he uses a cane.

As early as 2010, Benedict began to look worn out: He had lost weight and didn't seem fully engaged when visiting bishops briefed him on their dioceses. But as tired as he often seemed, he would also bounce back, enduring searing heat in Benin to bless a child and gamely hanging on when a freak storm forced him to cut short a speech during a youth festival in Madrid in 2011.

His 89-year-old brother, Georg Ratzinger, said doctors recently advised the pope not to take any more trans-Atlantic trips.

"His age is weighing on him," Ratzinger told the dpa news agency in Germany. "At this age, my brother wants more rest."

"He has looked very, very run down," agreed U.S. Cardinal Edwin O'Brien, who was present for Monday's announcement, speaking to Sirius XM's "The Catholic Channel.

Benedict emphasized that to carry out the duties of being pope, "both strength of mind and body are necessary -- strengths which in the last few months, have deteriorated in me."

"After having repeatedly examined my conscience before God, I have come to the certainty that my strengths due to an advanced age are no longer suited" to the demands of being the pope, he told the cardinals.

In a way, it shouldn't have come as a surprise. Benedict himself raised the possibility of resigning if he were too old or sick to continue.

"If a pope clearly realizes that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right, and under some circumstances, also an obligation to resign," Benedict said in the 2010 book "Light of the World."

But he stressed that resignation was not an option to escape a particular burden, such as the sex abuse scandal.

"When the danger is great, one must not run away. For that reason, now is certainly not the time to resign. Precisely at a time like this one must stand fast and endure the situation," he said.

The Vatican newspaper, L'Osservatore Romano, said Benedict decided to resign after his March 2012 trip to Mexico and Cuba, an exhausting but exhilarating visit where he met with fellow-octogenarian Fidel Castro and was treated to a raucous and warm welcome.

Although popes are allowed to resign, only a handful has done it -- and none for a very long time.

The last pope to resign was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism, a dispute among competing papal claimants. The most famous resignation was Pope Celestine V in 1294; Dante placed him in hell for it.

There are good reasons why others haven't followed suit, primarily because of the fear of a schism with two living popes. Lombardi sought to rule out such a scenario, saying church law makes clear that a resigning pope no longer has the right to govern the church.

When Benedict was elected in 2005 at age 78, he was the oldest pope chosen in nearly 300 years. At the time, he had already been planning to retire as the Vatican's chief orthodoxy watchdog to spend his final years writing in the "peace and quiet" of his native Bavaria.

On Monday, Benedict said he plans to serve the church for the remainder of his days "through a life dedicated to prayer." The Vatican said after he resigns he will travel to Castel Gandolfo, the papal summer retreat south of Rome, and then live in the monastery.

All cardinals under age 80 are allowed to vote in the conclave, the secret meeting held in the Sistine Chapel where cardinals cast ballots to elect a new pope. As per tradition, the ballots are burned after each voting round; black smoke that snakes out of the chimney means no pope has been chosen, while white smoke means a pope has been elected.

There are currently 118 cardinals under age 80 and thus eligible to vote, 67 of them appointed by Benedict. However, four will turn 80 before the end of March. Depending on the date of the conclave, they may or may not be allowed to vote.

Benedict in 2007 passed a decree requiring a two-thirds majority to elect a pope, changing the rules established by John Paul in which the voting could shift to a simple majority after about 12 days of inconclusive balloting. Benedict did so to prevent cardinals from merely holding out until the 12 days had passed to push through a candidate who had only a slim majority.

Contenders to be Benedict's successor include Cardinal Angelo Scola, archbishop of Milan; Schoenborn, the archbishop of Vienna, and Cardinal Marc Ouellet, the Canadian head of the Vatican's office for bishops.

Longshots include Cardinal Timothy Dolan of New York. Although Dolan is popular and backs the pope's conservative line, being from a world superpower would probably hurt his chances. That might also rule out Cardinal Raymond Burke, an archconservative and the Vatican's top judge, even though he is known and respected by most Vatican cardinals.

Cardinal Antonio Tagle, the archbishop of Manila, has impressed many Vatican watchers, but at 56 he is considered too young.

Cardinal Peter Kodwo Appiah Turkson of Ghana is one of the highest-ranking African cardinals at the Vatican, currently heading the Vatican's office for justice and peace, but he's something of a wild card.

There are several "papabiles" in Latin America, though the most well-known -- Cardinal Oscar Andres Rodriguez Maradiaga of Honduras -- is considered far too liberal to be elected by such a conservative College of Cardinals.

Whoever it is, he will face a church in turmoil: The sex abuse scandal has driven thousands of people away from the church, particularly in Europe. Rival churches, particularly evangelical Pentecostal groups in the developing world, pose new competition. And as the pope himself has long lamented, many people in an increasingly secular world simply believe they don't need God.

The timing of Benedict's announcement was significant: Lent begins this week on Ash Wednesday, the most solemn period on the church's calendar that culminates with Holy Week and Easter on March 31. It is also the period in which the world witnessed the final days of John Paul's papacy in 2005.

The timing means that there will be a spotlight cast on Cardinal Gianfranco Ravasi, the Italian head of the Vatican's culture office who has long been on the list of "papabile." Benedict selected him to preside over the Vatican's spiritual exercises during Lent.

And by Easter Sunday, the Catholic Church will almost certainly have a new leader, Lombardi said -- a potent symbol of rebirth in the church on a day that celebrates the resurrection of Christ.

Daniela Petroff contributed from Vatican City, Thomas Adamson from Paris and Philipp-Moritz Jenne in Vienna.