Pope Benedict XVIsurprised the world and left the Catholic church stunned when he said on Monday thathe would resign– the first pope to do so since the middle ages.

The move, announced without warning, will take place on 28 February and leave the papacy vacant until a successor is chosen.

AVaticanspokesman said Benedict's aides were "incredulous" when he told them he would step down because he was too weak to fulfil his duties. He summoned a meeting of cardinals to tell them of "a decision of great importance for the life of the church".

One of those called to hear the announcement, the Mexican prelate Monsignor Oscar Sanchez, said none of the cardinals had expected it. "The pope took a sheet of paper and read from it. He just said that he was resigning and that he would be finishing on February 28," he said.

"The cardinals were just looking at one another. Then the pope got to his feet, gave his benediction and left. It was so simple; the simplest thing imaginable. Extraordinary. Nobody expected it. Then we all left in silence. There was absolute silence … and sadness."

His successor is expected to be elected by the end of March and possibly for the beginning of holy week on 24 March. Benedict will honour public commitments and engagements until the date of his resignation, after which he will move to a summer residence near Rome and then to a former monastery within Vatican territory.

He will take no part in the process to elect a successor. Cardinals will meet and vote on nominees in a series of ballots until a new pope is chosen.

Federico Lombardi, a Vatican spokesman, said Benedict, 85, had resigned not because of "difficulties in the papacy" or a specific illness but instead a progressive decline in his strength.

"In the last few months he has seen a decline in vigour, both of the body and spirit," Lombardi told reporters. "It was his personal decision taken with full freedom, which deserves maximum respect."

In a statement, Benedict said: "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.

"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of bishop of Rome, successor of Saint Peter, entrusted to me by the cardinals on 19 April 2005, in such a way that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a conclave to elect the new supreme pontiff will have to be convoked by those whose competence it is."

Benedict, who became the 265th pope in 2005, has arthritis, particularly in his knees, hips and ankles. He had been due to travel to Brazil, the largest Catholic country in the world, in July for a youth festival, but concerns had been raised among Vatican observers about whether he was well enough.

A voluntary papal resignation is rare – certainly in recent centuries. Pope Celestine V exercised his right to abdicate in 1294.

Pope Benedict said Monday that gay marriage was one of several threats to the traditional family that undermined "the future of humanity itself."

The pope made some of his strongest comments against gay marriage in a new year address to the diplomatic corps accredited to the Vatican in which he touched on some economic and social issues facing the world today.

He told diplomats from nearly 180 countries that the education of children needed proper "settings" and that "pride of place goes to the family, based on the marriage of a man and a woman."

"This is not a simple social convention, but rather the fundamental cell of every society. Consequently, policies which undermine the family threaten human dignity and the future of humanity itself," he said.

The Vatican and Catholic officials around the world have protested against moves to legalize gay marriage in Europe and other developed parts of the world.

One leading opponent of gay marriage in the United States is New York Archbishop Timothy Dolan, whom the pope will elevate to cardinal next month.

Dolan fought against gay marriage before it became legal in New York state last June, and in September he sent a letter to President Barack Obama criticizing his administration's decision not to support a federal ban on gay marriage.

In that letter Dolan, who holds the powerful post of president of the U.S. Bishops Conference, said such a policy could "precipitate a national conflict between church and state of enormous proportions."

The Roman Catholic Church, which has some 1.3 billion members worldwide, teaches that while homosexual tendencies are not sinful, homosexual acts are, and that children should grow up in a traditional family with a mother and a father.

"The family unit is fundamental for the educational process and for the development both of individuals and states; hence there is a need for policies which promote the family and aid social cohesion and dialogue," Benedict told the diplomats.

Gay marriage is legal in a number of European countries, including Spain and the Netherlands.

Some Churches that have allowed gay marriage, women priests, gay clergy and gay bishops have been losing members to Catholicism, and the Vatican has taken steps to facilitate their conversion.

In 2009, Benedict decreed that Anglicans who leave their Church, many because they feel it has become too liberal, can find a home in Catholicism in a parallel hierarchy that allows them to keep some of their traditions.

The Vatican has since set up "ordinariates," structures similar to dioceses, in Britain and the United States to oversee ex-Anglicans who have converted and be a point of contact for those wishing to do so.

VATICAN CITY (AP) — Pope Benedict XVI insisted on Saturday that all of society's institutions and not just the Catholic churchmust be held to "exacting" standards in their response to sex abuse of children, and defended the church's efforts to confront the problem.

Benedict acknowledged in remarks to visiting U.S. bishops during an audience at the Vatican that pedophilia was a "scourge" for society, and that decades of scandals over clergy abusing children had left Catholics in the United States bewildered.

"It is my hope that the Church's conscientious efforts to confront this reality will help the broader community to recognize the causes, true extent and devastating consequences of sexual abuse, and to respond effectively to this scourge which affects every level of society," he said.

"By the same token, just as the church is rightly held to exacting standards in this regard, all other institutions, without exception, should be held to the same standards," the pope said.

An official of a U.S. group advocating for victims of clergy abuse lamented that Benedict, with his remarks, was setting a "terrible example" for bishops.

"No public figure talks more about child safety but does little to actually make children safer than Pope Benedict," David Clohessy, national director of the Survivors Network of those Abused by Priests, told The Associated Press in an emailed statement.

"The pope would have us believe that this crisis is about sex abuse. It isn't. It is about covering up sex abuse," Clohessy said. "And while child sex crimes happen in every institution, in no institution are they ignored or concealed as consistently as in the Catholic church."

The pedophile scandal has exploded in recent decades in the United States, but similar clergy sex abuse revelations have tainted the church in many other countries, including Mexico, Ireland, and several other European nations, including Italy.

But the most high-profile sex abuse case in the United States at the moment doesn't involve the church. Penn State university's former defensive football coordinator Jerry Sandusky has been charged with sexually abusing eight boys, and the fallout has led to the firing of longtime coach Joe Paterno and the departure of university president Graham Spanier.

College football in the U.S. is highly popular. The scandal has shaken the reputation of a college program that long had prided itself on integrity.

An advocacy group for those who have been sexually abused cited the Penn State scandal in its scathing criticism of the pope.

"It takes hubris for Pope Benedict to tell his bishops that the Catholic Church has led in the fight against sexual abuse of children," said Kristine Ward, chair of the National Survivor Advocates Coalition. "Issuing self-satisfied pats on the back while children remain in danger only further diminishes the church's credibility and deepens the laryngitis in its moral voice."

"The church to this day, while waving a moral flag, hasn't even come close to the Penn State Board of Trustees response — no bishop has been fired," Ward said in a statement.

Benedict didn't address accusations by many victims and their advocates that church leaders, including at the office in the Vatican that Benedict headed before becoming pontiff, systematically tried to cover up the scandals, and that they have rarely been held accountable for that.

Investigations, often by civil authorities, revealed that church hierarchy frequently transferred pedophile priests from one parish to another.

Benedict told the bishops that his papal pilgrimage to the United States in 2008 "was intended to encourage the Catholics of America in the wake of the scandal and disorientation caused by the sexual abuse crisis of recent decades."

Echoing sentiment he has expressed in occasional meetings with victims of the abuse on trips abroad, Benedict added: "I wish to acknowledge personally the suffering inflicted on the victims and the honest efforts made to ensure both the safety of our children and to deal appropriately and transparently with allegations as they arise."

Benedict seemed to be reflecting some churchmen's contentions that the church has wrongly been singled out as villains for the abuse, a view that angered victims' advocates.

"The pope is again setting a terrible example for the world's bishops, echoing the claim by some of them that the church hierarchy is somehow being picked on by the public, the press and their parishioners," Clohessy said .

Despite criticism over U.S. bishops' handling of the abuse scandals, Benedict exhorted the churchmen to be moral compasses for U.S. society. The bishops, in Rome for consultations with the pope that are scheduled every five years, were urged to speak out "humbly yet insistently in defense of moral truth."

Benedict lamented what he called efforts to stop the church from speaking out publicly.

Earlier this month, U.S. Roman Catholic bishops vowed to defend their religious liberty in the face of growing acceptance of gay marriage and what they called attempts by secularists to marginalize faith.

In Illinois, for example, government officials ceased working with Catholic charities on adoptions and foster-care placement because the religious agencies refuse to recognize a new civil union law. Illinois bishops are suing the state.

Bishops have also pressed federal officials for broader religious exception to U.S. President Barack Obama's health care overhaul, which mandates that private insurers to pay for contraception.

"Despite attempts to still the church's voice in the public square, many people of good will continue to look to her for wisdom, insight and sound guidance in this far-reaching crisis," Benedict said, citing what he called a "growing sense of dislocation and insecurity" in the face of economic woes.

But he acknowledged that some of the bishops' own flock are turning away from the church, which he blamed on effects of a "secularized culture." Many U.S. Catholics shun Sunday Mass attendance or disregard such Vatican positions against contraception and divorce.

In his traditional Christmas address yesterday to cardinals and officials working in Rome, Pope Benedict XVI claimed that pedophilia wasn't considered an absolute evil as recently as the 1970s and that child pornography is increasingly considered "normal" by society.

"In the 1970s, pedophilia was theorized as something fully in conformity with man and even with children," the Pope said.

"It was maintained - even within the realm of Catholic theology - that there is no such thing as evil in itself or good in itself. There is only a 'better than' and a 'worse than'. Nothing is good or bad in itself."

The Pope said abuse revelations in 2010 reached "an unimaginable dimension" which brought "humiliation" on the Church.

The Pope called on senior clerics "to repair as much as possible the injustices that occurred" and to help victims heal through a better presentation of the Christian message.

"We cannot remain silent about the context of these times in which these events have come to light," he said, citing the growth of child pornography "that seems in some way to be considered more and more normal by society" he said.

Pope Benedict also said sex tourism in the Third World was "threatening an entire generation".

"Catholics should be embarrassed to hear their Pope talk again and again about abuse while doing little or nothing to stop it and to mischaracterize this heinous crisis," said Barbara Blaine, the head of SNAP, the Survivors Network of those Abused by Priests,

"It is fundamentally disturbing to watch a brilliant man so conveniently misdiagnose a horrific scandal."

"The Pope insists on talking about a vague 'broader context' he can't control, while ignoring the clear 'broader context' he can influence - the long-standing and unhealthy culture of a rigid, secretive, all-male Church hierarchy fixated on self-preservation at all costs. This is the 'context' that matters."