Pope Benedict XVI will say Mass in Glasgow this evening as he continues his visit to Britain, the first ever state visit by a pope there. Earlier today, the pontiff urged the country to protect its Christian roots and tradition of tolerance in the face of threats from aggressive secularism and atheist extremism.
More than 100,000 people, including protesters, turned out to watch the 83-year-old as he was driven through the streets of Edinburgh this morning wearing a green plaid scarf.
The Pope today criticised Catholic Church leaders as 'insufficiently vigilant' during decades of sexual abuse of children.
He also met Britain's Queen Elizabeth II at Holyrood Palace, the Queen's official residence in Scotland.
The Pope's four-day visit also includes events in London and Birmingham.
Groups that plan to protest against the pope's trip include atheists, secular organisations, and those who want the pope to be held legally responsible for the Roman Catholic Church sexual abuse scandals.
Speaking to reporters on the plane taking him to Scotland, Benedict acknowledged Catholic leaders had been 'insufficiently vigilant' during decades of sexual abuse of children.
In some of his clearest remarks on the scandal, he told reporters aboard the plane taking him to Scotland that he was shocked by what he called 'a perversion' of the priesthood.
He said: 'These revelations were a shock for me, a great sadness. It is difficult to understand how this perversion of the priestly ministry was possible.'
'It is also a great sadness that the authority of the church was not sufficiently vigilant and not sufficiently quick and decisive in taking the necessary measures.'
The pope also reminded Britons to beware extremism, saying that the attempt by totalitarian regimes in the 20th century to eliminate God should provide 'sobering lessons' on tolerance.
'Today, the United Kingdom strives to be modern and multicultural society. In this challenging enterprise, may it always maintain its respect for those traditional values and cultural expressions that more aggressive forms of secularism no longer value or even tolerate,' he said.
'Let it not obscure the Christian foundation that underpins its freedoms,' he said.
Preparations for the visit have been overshadowed by a papal advisor's remarks likening Britain to a third world country and claiming it discriminates against Christians.
The Vatican has distanced itself from the remarks and says that ill health has forced Cardinal Walter Kasper to withdraw from the Papal party.
The turnout at this evening's Papal mass in Glasgow will be the first significant barometer of the pontiff's popularity.
Ian Paisley this morning led dozens of people in a protest against the Papal visit.
Members of the Free Presbyterian Church unveiled a banner near the Magdalen Chapel in Cowgate and distributed booklets outlining their opposition to the Pope's trip.
The booklet stated that 'recent scandals' within the Roman Catholic Church meant that the Pope would not receive a universal welcome.
Speaking at the same time as the Pope was in Edinburgh, Mr Paisley said: 'I don't want his blessing and I will be keeping as far away as I can.
Meanwhile, some victims' groups have come out against any private meetings with the pope during the four-day visit.