(ANSA) - Vatican City, February 11 - Pope Benedict XVI shocked the world on Mondayby announcing that he will step down as the head of the Catholic Church as of February 28, citing his 'advanced age' as the reason and spurring speculation the world's 1.1 billion Catholics may get an African as their next leader.

  The 85-year-old German pontiff said he was "well aware of the seriousness of this act" but felt he lacked the necessary mental and physical strength to continue.

 "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 an adequate exercise of the Petrine ministry," Benedict told a meeting of cardinals.

 "I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering.

"However, in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me".

Benedict is the first pope to resign in modern times and Monday's announcement stunned the world.

Vatican spokesman Father Federico Lombardi said even he was taken aback by Benedict's decision.

"The pope took us a bit by surprise," Lombardi told a press conference.

He added that recently those near the pontiff had noticed "he had become more tired and fatigued than in the past".

Italian cardinal Angelo Sodano, the dean of the College of Cardinals, said that Pope Benedict XVI's announcement was a "bolt out of the blue".

Italian Premier Mario Monti was stunned."I am very shocked by this unexpected news," Monti said. Monti said he had no "signs or hints" of the impending decision during recent meetings with the pope.

At 78, the former Cardinal Joseph Ratzinger was one of the oldest new popes in history when he was elected in 2005 following the death of John Paul II.  

Benedict said the pontificate would be vacant as of 20.00 Italian time (19:00 GMT) of February 28 and that he had called a conclave to elect his successor. 

"Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects," Benedict told the cardinals.

"And now, let us entrust the Holy Church to the care of Our Supreme Pastor, Our Lord Jesus Christ, and implore his holy Mother Mary, so that she may assist the Cardinal Fathers with her maternal solicitude, in electing a new Supreme Pontiff.

 "With regard to myself, I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer".

WORLD LEADERS SHOCKED AND MOVED.

Around the world, stunned religious leaders and heads of state paid tribute to Benedict.

Chief Imam for the Sunni muslims, Ahmad el Tayyeb, expressed "shock" at the news from the margins of a closed door meeting in Cairo to elect a new grand Mufti of Egypt.

Chief rabbi for Israel, Yona Metzger, credited Pope Ratzinger with forging "the best relations between the Rabbinate and the Catholic Church" and wished the pope good health and a long life.

"He must be given credit for having done a great deal for inter-religious ties in the world between Christianity, Judaism and Islam," Metzger added.

"It is moving news," said German Chancellor Angela Merkel, commenting on the German pope's decision, and a choice that elicits "my greatest respect".

Cardinal Keith O'Brien, head of the Scottish Catholic church, said he was "shocked" and "heartbroken" by the news, and sent a message throughout the Scottish parishes to pray for Joseph Ratzinger in his moment of difficulty.

French president Francois Holland called the pope's decision "highly respectable" and a "human decision".

British Prime Minister David Cameron wished the pope well and gave him credit for tireless work to reinforce ties between Britain and the Catholic Church.

BOOKIES SAY IT COULD BE AFRICA'S TIME.

The pope's resignation announcement stunned the world and triggered a flurry of rumours and speculation about who will replace the pontiff.

With his replacement not likely to be selected until the end of March, bookies, politicians, and Vatican watchers will have plenty of time to weigh candidates.

But as the Catholic Church rises in size and power in Africa, as well as in Asia and Latin America, pressure is also increasing for a non-European pope to be selected by the church's cardinals, who will meet in conclave in mid-March.

That helps explain why online betting agency Agimeg says that the next pope could be Nigerian Cardinal Francis Arinze, 80, and in second place Cardinal Peter Turkson of Ghana, 64. Arinze, known as a scholarly conservative, came to Rome to study theology at the Pontifical Urban University, where he ultimately earned a doctorate in sacred theology summa cum laude.

But at popular Dublin-based bookmaker Paddy Power Plc, Turkson of Ghana is the favorite at odds of 9-4 to replace Pope Benedict, and Arinze is in third place at 3-1. Turkson, who lives in Rome and speaks numerous languages, was educated in the United States, attending St. Anthony-on-Hudson Seminary in Rensselaer, New York, where he obtained a Bachelor's degree in theology.

Turkson was appointed president of the Pontifical Council for Justice and Peace by Pope Benedict in 2009.

Canada's Marc Ouellet, 68, is second favorite at 5-2, according to Paddy Power.

However, his election seems less likely.

Ouellet, who became a cardinal in 2003, was quoted in June 2011 as suggesting he would have no interest in the Pope's job because its intensely heavy workload "would be a nightmare" with crushing responsibilities.

"It's the kind of thing you don't campaign for".

Bookies have named Milan Cardinal Angelo Scola as the European favourite to succeed Benedict, followed by Vatican Secretary of State Tarcisio Bertone and Cardinal Angelo Bagnasco, archbishop of Genoa and president of the Italian Bishops' Conference.

The name to be chosen by the next pope is also subject for wagering, with Peter in the lead, followed by Pius, John Paul, John, and Benedict.