EDINBURGH, Scotland (AP) — Pope Benedict XVI waded into the hostile atmosphere Thursday of highly secular Britain, admitting the Catholic Church did not act decisively or quickly enough to remove priests who molested children in his strongest comments yet on the worldwide sex abuse crisis shaking his church.

In a visit unprecedented for the bitter opposition to his papacy, Benedict warned against "aggressive forms" of secularism. The German pope recalled how Britain had stood against "Nazi tyranny that wished to eradicate God from society."

Benedict's historic four-day state visit has been overshadowed by disgust over the abuse scandal and indifference in Britain, where Catholics are a minority of 10 percent and endured centuries of bloody persecution and discrimination until the early 19th century.

The trip is the first state visit by a pope to the U.K., and his meeting with Queen Elizabeth II was symbolically significant because of the historic divide between the officially Protestant nation and the Catholic Church.

Only 65,000 of the faithful had tickets to an open-air Mass at Bellahouston Park in Glasgow, far less than the 100,000 initially expected. The British media has been particularly hostile to the pope's visit, noting its $18.7 million (12 million pound) security cost to taxpayers at a time of austerity measures and job losses.

Many in Britain are also strongly opposed to Benedict's hard line against homosexuality, abortion and using condoms to prevent the spread of AIDS. Protests were planned and "Pope Nope" T-shirts were spotted around London.

Still, a crowd of 125,000 in Edinburgh welcomed Benedict warmly as his Popemobile paraded through the streets, with cheers on Princes Street heard from a mile away and well-wishers waving the Holy See's yellow-and-white flag.

Only around 20 protesters gathered at the designated demonstration spot in Glasgow, where the pope celebrated an afternoon Mass, their complaints ranging from abusive priests to the pope's stance on contraception and homosexuality.

While flying to Britain, Benedict was asked about polls that suggest many Catholics have lost trust in the church as a result of the sex abuse scandals. Benedict said he was shocked and saddened about the scope of the abuse, in part because priests take vows to be Christ's voice upon ordination.

"It's difficult to understand how a man who has said this could then fall into this perversion. It's a great sadness," Benedict said in Italian. "It's also sad that the authority of the church wasn't sufficiently vigilant, and not sufficiently quick or decisive to take necessary measures" to stop it.

He said victims were the church's top priority as it tries to help them heal spiritually and psychologically.

"How can we repair, what can we do to help these people overcome this trauma, find their lives again and find again the trust in the message of Christ?" Benedict said.

He insisted that abusive priests must never again be allowed access to young children, saying they suffer from an illness that "goodwill" cannot cure. In addition, he said, candidates for the priesthood must be better screened.

The crowds that turned out in Scotland were enthusiastic.

"I've brought my wee girl Laura to see the pope," said James Hegarty, a 42-year-old unemployed Edinburgh resident. "She's only 4, but it's a once in a lifetime chance to see him."

Tens of thousands waved flags and applauded as Benedict arrived in his Popemobile for the Mass in Glasgow. At one point, he rolled down the vehicle's window to kiss a baby dressed all in pink.

Scottish singing sensation Susan Boyle warmed up the crowd and then fulfilled a dream of singing before the pope, serenading him with "Make Me a Channel of Your Peace."

In his homily, the 83-year-old pope warned young people against the temptations posed by drugs, money, sex, pornography and alcohol, "which the world tells you will bring you happiness."

The pope's first meeting of the day was with Queen Elizabeth II, both the head of state and head of the Church of England, at the Palace of Holyroodhouse in Edinburgh, chosen because she spends her summers at Balmoral Castle in Scotland.

The medieval castle is the queen's official residence in Scotland and a potent symbol of the historic divisions that separate Catholics and Anglicans. Mary Queen of Scots lived there during her brief reign as the Catholic queen of Scotland before her execution in 1587 by King Henry VIII's daughter, Queen Elizabeth I.

Henry had broken with Rome earlier in the 16th century, a division followed by centuries in which Catholics were fined, discriminated against and killed for their faith in Britain.

At their meeting Thursday, the queen told Benedict his visit reminded all Britons of their common Christian heritage and said she hoped relations between the Anglican Church and the Catholic Church would be deepened as a result. Efforts for closer relations between Catholics and Anglicans have been clouded recently by Benedict's overtures to conservative Anglicans.

The queen also praised the Catholic Church's "special contribution" to helping the poorest and most vulnerable people around the world.

The pope, too, recalled the shared Christian heritage of Catholics and Anglicans and said he wanted to extend a "hand of friendship" to the British people during his trip.

Tartan-wearing bagpipers marched and thousands of people watched under blustery, cloud-streaked blue skies. The pontiff himself donned a green tartan scarf as he rode through Edinburgh.

Later, he enjoyed a very Scottish treat: a lunch of haggis — sheep heart, liver and lungs simmered in sheep stomach — at the home of Scottish Cardinal Keith O'Brien.

The last papal visit to Britain was by John Paul II in 1982, Benedict's predecessor who was treated like a superstar and drew a crowd of 250,000 for Mass at the same Glasgow park. The bookish Benedict appears uncomfortable before large crowds.

The pope's visit coincides with the 450th anniversary of the Reformation in Scotland.

In Edinburgh, about 80 protesters led by Northern Ireland Protestant leader the Rev. Ian Paisley gathered at the Magdalen Chapel where John Knox, the leader of the Scottish Reformation, preached.

"This visit should never have happened. We stand here against these abusers. This is a waste of taxpayers' money," Paisley said.

Benedict acknowledged the opposition in his airborne comments to reporters, saying Britain had a "great history of anti-Catholicism. But it is also a country with a great history of tolerance."

The Vatican has been reeling for months as thousands of victims around the globe have spoken out about priests who molested children, bishops who covered up for them and Vatican officials who turned a blind eye to the problem for decades. In the latest admission, hundreds of victims came forward in Belgium with tales of horrific abuse linked to at least 13 suicides.

Previously, Benedict has admitted the scandal was borne of "sins within the church" but he had never acknowledged in such detail the church's failure to act. Advocates for victims have long insisted he take more personal responsibility, given that he was in charge of the Vatican office that handled sex abuse cases and was archbishop of Munich when a pedophile priest was assigned pastoral work while undergoing therapy for having abused young boys.

The main U.S. victim's group dismissed Benedict's comments Thursday, noting that the only real action the Vatican has taken has been to tell bishops to report abuse to police if local laws require them to do so.

"Bishops across the world continue to deliberately choose secrecy and deception over safety and honesty in child sex cases," said Joelle Casteix of the Survivors Network of those Abused by Priests.

Vatican officials haven't confirmed that Benedict will meet with abuse victims while in Britain, but U.K. organizers say arrangements are being made.

After the Glasgow Mass, Benedict was traveling to London.

The highlight of his visit is the beatification Sunday of Cardinal John Newman in Birmingham, which will see the 19th-century English philosopher and Anglican convert take a step on his way to sainthood.

The Humanist Society of Scotland placed billboards between Edinburgh and Glasgow that read: "Two million Scots are good without God." It also took exception to the pope's comment Thursday about the Nazis.

"The notion that it was the atheism of Nazis that led to their extremist and hateful views or that somehow fuels intolerance in Britain today is a terrible libel against those who do not believe in God," the group said.

The Rev. Federico Lombardi, the Vatican spokesman, responded that Benedict — who was forced to become part of the Hitler Youth — chose his words wisely. "You can agree or not, but I think the pope knows very well what the Nazi ideology was," Lombardi said.

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Winfield reported from Edinburgh and Simpson from London. Associated Press reporters Ben McConville in Edinburgh and Danica Kirka in London contributed to this report.