Pope Benedict XVI has expressed his "deep sorrow" for the "unspeakable crimes" of child abuse within the Catholic Church.
The strongest public apology yet over the scandal was offered during a Mass at Westminster Cathedral on the London leg of his four-day state visit.
The Pope then conducted a prayer vigil with thousands of people in Hyde Park.
Later it was confirmed that the Pope had met five people who had suffered abuse at the hands of the clergy.
Speaking to hundreds of members of the Catholic Church, including senior clergy, the Pope said he was gravely concerned by "the immense suffering" caused by the abuse, especially by Church ministers.
He also acknowledged "the shame and humiliation" caused by "these sins".
"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," he told the congregation.
He 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".
"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 it was confirmed that the Pope had held a private 30-minute meeting at the Vatican ambassador's home in Wimbledon with five abuse survivors, three of whom were from Yorkshire, one from London and another from Scotland.
The Vatican said during the intensely emotional meeting, which began and ended with prayers, the Pope expressed his "deep sorrow and shame" to the four women and one man.
Bill Kilgallon, chairman of the National Catholic Safeguarding Commission, said he spoke to the abuse survivors beforehand and they were "understandably quite emotional".
"I would not say they were angry but I think there's anger in them," he said.
"I think that's a really positive thing because what happened was a real betrayal of trust."
Later, representatives from the safeguarding commission, including Mr Kilgallon, met the Pope in a separate meeting in Vauxhall.
Commenting on the Pope's message during the Westminster Mass, Peter Isely, of the Survivors Network of those Abused by Priests, said: "We don't need a Pope who is sad about crimes. We need a Pope who will prevent crimes. And his words prevent nothing."
Peter Saunders, chief executive of the National Association for People Abused in Childhood, who was also a victim himself, called on the Pope to make funding available to abuse survivors and provide access to Vatican files.
During the afternoon a march and rally organised by Protest the Pope - an umbrella campaign group opposed to the Pope's visit - took place in central London.
Organisers said 20,000 people took part but the police said they were unable to confirm numbers.
Gay rights campaigner Peter Tatchell, who was on the march, said if he had the chance to speak to the Pope, he would ask him to hand over any secret Vatican sex abuse files and co-operate with the police.
"The Pope keeps apologising but he only apologises for the failings of others. He is not a person who deserves the honour of a state visit," he said.
In the evening, tens of thousands of people took part in an open-air prayer vigil in London's Hyde Park.
As darkness fell, pilgrims waved Papal standards and candles as the Pope read prayers from a vast stage surrounded by choirs and clergy.
Teenage Britain's Got Talent singer Liam McNally performed a solo before the Pope departed to much applause.
Earlier engagements included a meeting with Prime Minister David Cameron, his deputy Nick Clegg and acting Labour leader Harriet Harman. The former prime minister Tony Blair, a convert to Catholicism, attended the Mass at Westminster.
There were also opportunities for the Pope to meet the public, including the young and old.
Thousands of young Catholics greeted him to cheers outside the cathedral. Among them was Paschal Uche, 21, who told the Pope: "Your visit brings us together. It is like a family reunion and we are very, very pleased to see you."
Later he visited St Peter's Residence for Older People in Vauxhall, south London, where the 83-year-old said he had experienced the "joys and the struggles" of age.
"The provision of care for the elderly should be considered not so much as an act of generosity as the repayment of a debt of gratitude," he said.
The Pope added that the growing number of old people was a "blessing" for society and that every generation could learn from the experience and wisdom of the generation that preceded it.
The day also saw the Pope speak of his "deep love" for the Welsh people and deliver a blessing to the nation in Welsh.
The Pope's schedule is continuing as planned, despite six men being held in connection with an alleged terrorist threat to his visit. Sources at Scotland Yard told the BBC that detectives have interviewed the men and believe there was "no credible threat" to the Pope.
The street cleaners for Veolia Environment Services, a cleaning company working for Westminster Council, were arrested on Friday after being overheard in the works canteen apparently plotting an attack.
Police have refused to confirm reports that they were joking, and say they had to investigate what might have been a genuine threat.
The men can be held until early on Sunday before police must apply for an extension.