Pope Benedict apologized to victims of sexual abuse Saturday, saying paedophile priests had brought “shame and humiliation” on him and the entire Roman Catholic Church. At the start of his third day in Britain, the pope celebrated a Mass for some 2,000 people in Westminster Cathedral, the mother church for Roman Catholics in England and Wales and a symbol of the struggle of Catholics to assert their rights after the Reformation. It was the 83-year-old pontiff's latest attempt to come to grips with the scandal that has rocked the 1.1 billion-member Church, particularly in Europe and the United States. A crowd of thousands, the largest of the trip so far, began a march from Hyde Park to Downing Street, the prime minister's official residence, to protest the visit. “I think of the immense suffering caused by the abuse of children, especially within the Church and by her ministers. Above all, I express my deep sorrow to the innocent victims of these unspeakable crimes ...,” he said in his sermon in the towering cathedral built in the late 19th century. “I also acknowledge with you the shame and humiliation that all of us have suffered because of these sins,” he said, adding that he hoped “this chastisement” would contribute to the healing of the victims and the purification of the Church. He has apologized before for sexual abuse by priests and has acknowledged that the Church was slow to deal with the problem. But his comments on Saturday were among his most direct. Still, victims groups said they were not satisfied, with one named Bishops Accountability calling it “public relations not penitence.” The Pope, who flies to Birmingham in central England on Sunday, began his last day in London by holding separate meetings with Prime Minister David Cameron, deputy Prime Minister Nick Clegg and acting opposition leader Harriet Harman. It was during Harman's tenure as a minister in the previous Labor government that the Pope condemned an equality bill going through parliament that would have forced churches to hire homosexuals or transsexuals. The provision was later defeated.