Pope Benedict XVI said Saturday he was ashamed of the "unspeakable" sexual abuse of children by priests, telling the British faithful during mass in Westminster Cathedral that he was deeply sorry and hoped the church's humiliation would help victims heal.

Pope Benedict XVI meets the crowd after Saturday's mass at Westminster Cathedral in London. (Akira Suemori/Associated Press)

Benedict also said he hoped the church would be able to use its contrition to purify itself of the "sins" of its ministers and renew its commitment to educating the young.

Benedict confronted the abuse scandal head-on during his homily, which was broadcast live on British television, a day after six people were arrested in an alleged terrorist plot against him. They remained in custody Saturday.

Ahead of the mass, about 10,000 people, including abuse survivors, marched through the streets of central London to denounce the papal visit in the biggest protest yet against Benedict's five-year papacy.

The sex abuse scandal has clouded Benedict's four-day state visit to the 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.

The pontiff issued his comments in the seat of English Catholicism amid speculation that he might meet with British abuse victims, and as abuse survivors and others opposed to his visit prepared a march Saturday afternoon in London's Hyde Park to demand more accountability.

"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.

"I also acknowledge with you the shame and humiliation that all of us have suffered because of these sins," he said. Benedict 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."

He asked the faithful to show concern for victims and solidarity with priests. Among those in the cathedral were former British Prime Minister Tony Blair, a recent convert to Catholicism, and his wife, Cherie.

'He got [the apology] just about right.'—Martin Brown, who was in the crowd outside the church

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."

On his way to Britain, Benedict acknowledged to reporters that the church had failed to act quickly or decisively enough to stop the abuse and prevent it from recurring.

Victims groups have dismissed such comments as hollow, saying they want the church to turn over information about suspected pedophiles in its ranks and take action, not words, to make children safer.

"We don't need a pope who is sad about crimes. We need a pope who will prevent crimes," Peter Isely of the Survivors Network of Those Abused by Priests, said in a statement after the pontiff's comments Saturday. "And his words prevent nothing."

After the church service, the Pope met with five people who were molested by priests as children and apologized to them. They talked and prayed together for about half an hour at the Vatican's apostolic nunciature in Wimbledon.

It was part of Benedict's busy third day in Britain, and a day after six men accused of plotting an attack during his visit were arrested.

Police staged a pre-dawn raid Friday on a garbage depot and arrested five street cleaners; a sixth person was arrested later in the day.

Former British prime minister Tony Blair, centre, his wife Cherie, left, and his son Euan, right, take part in a mass, presided over by Pope Benedict XVI at Westminster Cathedral in central London on Saturday. (Leon Neal/Associated Press)

Police say they were detained under the Terrorism Act "on suspicion of the commission, preparation or instigation of acts of terrorism."

The Vatican has said the Pope was informed of the arrests, was calm, and no changes to his itinerary were planned.

Benedict began his day by meeting with British Prime Minister David Cameron, Deputy Prime Minister Nick Clegg and Opposition Leader Harriet Harman.

After mass, he issued a special greeting to young believers and the Welsh faithful — singled out because he won't be travelling to Wales during this visit, only to England and Scotland.

He was scheduled to visit a home for the elderly before celebrating an evening prayer service in Hyde Park in preparation for Sunday's beatification of Cardinal John Henry Newman, a 19th century convert from Anglicanism whom the Pope wants to hold up as a model for the faithful.

On Friday, Benedict addressed Britain's political, financial and cultural elite in Westminster Hall, for centuries the seat of British political life. He demanded that religion have a voice in public policy and Christians in public roles be allowed to follow their consciences, lamenting that some even want to discourage Christmas celebrations.