In this photo provided by the Vatican newspaper L'Osservatore Romano, Mons. Franco Comaldo, left, a pope aide, looks at Pope Benedict XVI as he reads a document in Latin where he announces his resignation, during a meeting of Vatican cardinals, at the Vatican, Monday, Feb. 11, 2013. FILE - In this Sunday, March 25, 2012 file photo, pilgrims cheer and sing as they wait at the site where Pope Benedict XVI will give a Mass in Bicentennial Park near Silao, Mexico. AP Photo/Dario Lopez-Mills, File FILE - In this Sunday, March 25, 2012 file photo, Pope Benedict XVI waves from the popemobile wearing a Mexican sombrero as he arrives to give a Mass in Bicentennial Park near Silao, Mexico. AP Photo/Eduardo Verdugo, File FILE - In this April 28, 2009 file photo provided then by the Vatican newspaper L'Osservatore Romano, Pope Benedict XVI stands by the salvaged remains of Pope Celestine V, in the 13th-century Santa Maria di Collemaggio Basilica, the symbol of the city of L'Aquila, whose roof partially caved in during the quake. Benedict XVI announced Monday, Feb. 11, 2013 that he would resign on Feb. 28 because he was simply too infirm to carry on - the first pontiff to do so in nearly 600 years. Pope Celestine V, the 13th-century hermit and saint, was one of the earlier popes to resign. AP Photo/L'Osservatore Romano, file VATICAN CITY (AP) - Pope Benedict XVI said Monday he lacks the strength to fulfill his duties and on Feb. 28 will become the first pontiff in 600 years to resign. The announcement sets the stage for a conclave in March to elect a new leader for world's 1 billion Catholics. The 85-year-old pope announced the bombshell in Latin during a meeting of Vatican cardinals, surprising even his closest collaborators, even though Benedict had made clear in the past he would step down if he became too old or infirm to do the job. Benedict called his choice "a decision of great importance for the life of the church." Indeed, the move allows the Vatican to hold a conclave before Easter to elect a new pope, since the traditional mourning time that would follow the death of a pope doesn't have to be observed. It will also allow Benedict to hold great sway over the choice of his successor. He has already hand-picked the bulk of the College of Cardinals - the princes of the church who will elect the next pope - to guarantee his conservative legacy and ensure an orthodox future for the church. There are several papal contenders in the wings, but no obvious front-runner - the same situation when Benedict was elected pontiff in 2005 after the death of Pope John Paul II. The Vatican stressed that no specific medical condition prompted Benedict's decision, but in recent years, the pope has slowed down significantly, 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. His 89-year-old brother, Georg Ratzinger, said doctors had recently advised the pope not to take any more trans-Atlantic trips. "His age is weighing on him," Ratzinger told the dpa news agency. At this age my brother wants more rest. Benedict emphasized that carrying out the duties of being pope - the leader of more than a billion Roman Catholics worldwide - requires "both strength of mind and body." "In order to govern the bark of St. Peter and proclaim the Gospel, both strength of mind and body are necessary - strengths which in the last few months, have deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me," he said. Popes are allowed to resign; church law specifies only that the resignation be "freely made and properly manifested." But only a handful have done it. The last pope to resign was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism among competing papal claimants. The most famous resignation was Pope Celestine V in 1294; Dante placed him in hell for it. When Benedict was elected at age 78, he was the oldest pope chosen in nearly 300 years. At the time, he has 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 would serve the church for the remainder of his days "through a life dedicated to prayer." The Vatican said immediately after his resignation, Benedict would go to Castel Gandolfo, the papal summer retreat south of Rome, and then would live in a cloistered monastery. Contenders to be his successor include Cardinal Angelo Scola, archbishop of Milan, Cardinal Christoph 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, the general thinking is that the Catholic Church doesn't need a pope from a "superpower." Given half of the world's Catholics live in the global south, there will once again be arguments for a pope to come from the developing world. Cardinal Antonio Tagle, the archbishop of Manila, has impressed many Vatican watchers, but at 56 and having only been named a cardinal last year, 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. 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. The pontiff had been due to attend World Youth Day in July in Rio de Janeiro; by then his successor will have been named and will presumably make the trip. Benedict himself raised the possibility of resigning if he were simply too old or sick to continue on, when he was interviewed in 2010 for the book "Light of the World." "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. The former Cardinal Joseph Ratzinger had an intimate view as Pope John Paul II, with whom he had worked closely for nearly a quarter-century, suffered through the debilitating end of his papacy. The announcement took the Vatican - and the rest of the world - by surprise. Several cardinals on Monday didn't even understand what Benedict had said during the consistory, said the Rev. Federico Lombardi, the Vatican spokesman said. Others who did were stunned. "All the cardinals remained shocked and were looking at each other," said Monsignor Oscar Sanchez of Mexico who was in the room when Benedict made his announcement. Following John Paul's death in 2005, he was elected pope April 19 in one of the fastest conclaves in history, just about 24 hours after the voting began. Daniela Petroff contributed from Vatican City.