- Pope Benedict is on his second day in London and third in Britain
- He is meeting with Prime Minister David Cameron
- He will hold a Mass at Westminster Cathedral and a prayer vigil at Hyde Park
- Protesters are expected to rally against his visit
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 expressed his "deep sorrow" Saturday for the child sexual abuse scandal that has rocked the Catholic Church, the first time he has publicly addressed the issue on his four-day trip to Britain.
"I think of the immense suffering caused by the abuse of children, especially within the church and by her ministers," Benedict said during Mass at Westminster Cathedral. "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."
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 was due to greet both young and old -- the 2,500 children gathered in the cathedral's piazza and the residents of a Catholic care home nearby.
Thousands were also expected at London's Hyde Park for an afternoon prayer vigil 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.
Saturday was also a day of protests, however. Though demonstrators were on London's streets Friday as the pope drove past, a larger group including atheists, clerical sexual abuse survivors and gay rights campaigners planned a march to show their opposition to the pope's state visit.
Saturday was the third day of the pope's trip to Britain. He 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 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 coexist.
"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 and David Wilkinson contributed to this report.