- NEW: Advocacy group praises "courage" of victims who met with the pontiff
- Thousands of protesters demonstrate against Pope Benedict's visit
- Pope Benedict is on his second day in London and third in Britain
- He held a Mass at Westminster Cathedral and plans a prayer vigil at Hyde Park
As a sex abuse scandal rocked the Roman Catholic Church, what did Pope Benedict XVI -- then a cardinal and Vatican official -- know, and when? Watch the investigation "What the Pope Knew," September 25 & 26 at 8 p.m. ET on CNN U.S. and on September 25 at 7 p.m. CET and September 26 at 8 a.m. HK on CNN International.
London, England (CNN) -- Pope Benedict XVI met with five clergy abuse victims while on his official visit to the United Kingdom, the Catholic Communications Network said Saturday, the same day the pope expressed his "deep sorrow" for the scandal that has rocked the Catholic Church.
But his acknowledgement of the abuse suffered by children within the church -- the first time he has publicly addressed the issue during his four-day trip to Britain -- was not enough to dissuade thousands of protesters from expressing their anger on the streets of London.
Thousands gathered Saturday near Hyde Park, the site of an afternoon prayer vigil led by the pope for the beatification of British Cardinal John Henry Newman, a Catholic convert who died in 1890 and is credited with helping rebuild Britain's Catholic community.
A wide variety of sometimes expletive-laced signs could be seen dotting the crowd of demonstrators, which included atheists, clergy abuse victims and gay rights campaigners. One poster accused the pope of being the boss of the "world's largest sex gang." Another sign asked if he "fancied the baby Jesus."
The protests came hours after the pope addressed the abuse scandal during Mass at Westminster Cathedral.
"I think of the immense suffering caused by the abuse of children, especially within the church and by her ministers," he said. "Above all, 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.
"I also acknowledge with you the shame and humiliation which all of us have suffered because of these sins; and I invite you to offer it to the Lord with trust that this chastisement will contribute to the healing of victims, the purification of the church, and the renewal of her age-old commitment to the education and care of young people.
"I express my gratitude for the efforts being made to address this problem responsibly, and I ask all of you to show your concern for the victims and solidarity with your priests."
Later Saturday, the pope met with victims of clerical sexual abuse, according to the Catholic Communications Network, the media office of the Bishops' Conference. Three of the victims were from Yorkshire, one from London and one from Scotland, CCN said. The pope is also expected to meet Saturday with people involved in the National Catholic Safeguarding Commission at St. Peter's Residential Home in Vauxhall, London.
A board member of SNAP, the Survivors Network of those Abused by Priests, expressed hope that the outcome of the victims' meeting will be positive.
"We hope each of these brave individuals feels better as a result of the meeting, both now and years from now," Mark Serrano wrote in a statement. "It takes real courage to disclose your pain to others, especially those in authority. We hope the risk these victims have taken will prove to be fruitful, for them, for other victims and for children being molested today and in the future."
But Serrano also echoed other SNAP members' statements made earlier in the day that the pope needs to take action against abuse, not just make apologies for it.
"Today's meeting is more of the same from the Pope: all talk, no action," he wrote. "With literally the stroke of a pen, of course, he could radically change deeply-rooted, centuries-old destructive patterns of recklessness, callousness and deceit within the Catholic hierarchy that have directly led to hundreds of thousands of trusting children and vulnerable adults being raped, sodomized and fondled by clerics. But he refuses."
The Mass at Westminster Cathedral came on the second day of the pope's visit to London, and the third day of his visit to the United Kingdom.
Crowds lined the street outside the cathedral, the mother church for Catholics in England and Wales. Afterward, the pope greeted a crowd of 2,500 children gathered in the cathedral's piazza; later, he was due to visit the residents of a Catholic care home.
The pope spent Thursday in Edinburgh and Glasgow, Scotland, and planned to travel to Birmingham, England, on Sunday.
Six men remained in custody Saturday after their arrests a day earlier on terrorism charges -- incidents that prompted officials to review the pope's security arrangements.
Some news reports said the arrests involved a potential threat to the pope, but the Metropolitan Police declined to say whether the case was directly linked to the pontiff's visit.
Five of the men are street cleaners who were arrested before dawn on suspicion of the commission, preparation, or instigation of acts of terrorism. They appeared to be Algerian, a high-ranking source familiar with the investigation said, adding that some or all of them were probably in the country illegally.
A sixth man was arrested later in the day by counterterrorism detectives investigating the possible plot against the pope, police said.
All were arrested under the Terrorism Act 2000, which allows police to hold them without charge for 28 days.
Police said Saturday that their searches in the case were complete.
The arrests did not lead to any changes in the pope's schedule, which on Friday included events rich in history and symbolism. He met Archbishop of Canterbury Rowan Williams at his residence, Lambeth Palace -- the first time a pope has ever visited there.
Benedict then spoke to members of the British Parliament at Westminster Hall, which dates to 1097 and is the oldest building in the parliamentary complex. It was there in 1535 that Thomas More, a Catholic, was convicted of treason and sentenced to death for refusing to accept King Henry VIII's marriage annulment and repudiate the pope after Henry broke with the Vatican and created the Anglican Church.
The pontiff stressed to the political audience that reason and faith can and should co-exist.
"Religion," he said, "is not a problem for legislators to solve, but a vital contributor to the national conversation."
Later, at nearby Westminster Abbey, an Anglican church, the pope prayed alongside the archbishop of Canterbury at the tomb of Edward the Confessor, the English king who built the abbey and was buried there after his death in 1066. He spoke once again about the commitment to unity among Christian churches while noting the obstacles.
CNN's Melissa Gray, Carol Jordan and David Wilkinson contributed to this report.