LONDON - Pope Benedict XVI apologized Saturday to five people who were molested by priests as children in his latest effort to defuse the sex abuse crisis shaking his church, as thousands of people angered at the Vatican's response marched in central London in the biggest protest of his 5-year papacy.

Early Sunday, Scotland Yard said six men who had been detained on suspicion of plotting an attack on the pope had been freed after an investigation.

Benedict met for about 30-40 minutes with the victims - four women and a man from Scotland, England and Wales - at the Vatican's ambassador's residence in Wimbledon and expressed "his deep sorrow and shame over what the victims and their families suffered," according to the Vatican.

"He prayed with them and assured them that the Catholic Church is continuing to implement effective measures designed to safeguard young people, and that it is doing all in its power to investigate allegations, to collaborate with civil authorities and to bring to justice clergy and religious accused of these egregious crimes," it said.

Across town, abuse victims and demonstrators opposed to the pope's stance against homosexuality, abortion and using condoms to fight AIDS marched peacefully from Hyde Park to Downing Street, the major protest of Benedict's controversial four-day state visit.

They carried banners reading: "The pope is wrong - put a condom on" and "Pope protects pedophile priests."

Later Saturday, though, an estimated 80,000 people massed in Hyde Park cheering the pope as he celebrated an evening vigil.

The Vatican statement was similar to ones it issued after Benedict met with abuse victims over the past two years while visiting the United States, Australia and Malta. But continued revelations of abuse - the latest in Belgium - have failed to placate critics demanding that the pope and other Vatican officials take personal responsibility and crack down on bishops who covered up abuses by their clerics.

For the first time, Benedict also met with a group of professionals and volunteers who work to safeguard children and young people in church environments, Vatican spokesman the Rev. Federico Lombardi told reporters.

Bill Kilgallon, chairman of Britain's National Catholic Safeguarding Commission who helped organize the meeting, told the BBC that the victims got "something between 30 and 40 minutes."

Asked if the victims were angry, he said: "No, I wouldn't say they were angry. I think there is anger in them ... But anger can be very constructive if they work for change."

The sex abuse scandal has clouded Benedict's state visit to this deeply secular nation with a centuries-old history of anti-Catholic sentiment. Polls have indicated widespread dissatisfaction in Britain with the way Benedict has handled the crisis, with Catholics nearly as critical of him as the rest of the population.

Anger over the scandal runs high in Britain in part because of the enormous scale of the abuse in neighboring Ireland, where government reports have detailed systematic abuse of children at church-run schools and cover-up by church authorities.

During a Mass in Westminster Cathedral earlier Saturday, Benedict said he hoped the church's humiliation would help victims heal and help the church purify itself and renew its commitment to educating the young.

His comments, which were in line with his previous statements on the topic, were directed at Britain's Catholic community in the seat of the English church, a sign that Benedict wanted to speak to the faithful about the humiliation they all felt as Catholics.

"I express my deep sorrow to the innocent victims of these unspeakable crimes, along with my hope that the power of Christ's grace, his sacrifice of reconciliation, will bring deep healing and peace to their lives," Benedict said in his homily.

He acknowledged the shame and humiliation all the faithful had suffered as a result of the scandal and said he hoped "this chastisement will contribute to the healing of the victims, the purification of the church and the renewal of her age-old commitment to the education and care of young people."

Martin Brown, 34, who was in the crowd outside the cathedral, termed it "a good apology."

"He seemed to really mean it; he was genuinely sorry," Brown said. "It's good he mentioned it and it's good he didn't dwell on it for too long. He got it just about right."

The meeting with victims, part of a series of moves that began when he issued an apology during a meeting with reporters on his plane from Rome, took place near the famous tennis stadium in Wimbledon, a 30-minute ride on London's Underground from the protest march route from Hyde Park to Downing Street, near the British prime minister's residence.

Organizers said nearly 20,000 people - twice the number expected - took part. Scotland Yard took the unusual step of declining to put a figure on the crowd, saying it lacked manpower to make such an estimate.

Many wore rainbow-colored clothes or waved gay pride flags. Some members of the crowd bounced inflated condoms back and forth across the route.