GLASGOW (Reuters) - Pope Benedict started a trip to Britain on Thursday with some of the clearest criticism yet of his Church's handling of its sexual abuse crisis and urged the country to beware of "aggressive secularism."
Some 125,000 people, including a small number of protesters, watched the 83-year-old pope as he was driven through the Scottish capital Edinburgh wearing a green plaid scarf.
Hours before landing, he told reporters aboard the plane taking him to Scotland for a four-day trip to Britain that he was shocked by what he called "a perversion" of the priesthood.
"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," he added.
Advocates for victims have long been calling for Church leaders to assume more legal and moral responsibility for allowing the sexual abuse scandals to get out of hand in the United States and several countries in Europe.
Benedict has a delicate path to tread in England and Scotland in relations with the Anglican church after his offer last October making it easier for disaffected Anglicans, unhappy over the ordination of women and gay bishops, to convert.
These relations could be thrown into sharp focus on Friday when the pope is due to meet the Archbishop of Canterbury Rowan Williams, the head of the Church of England, the Anglican mother church, at Lambeth Palace, in London.
On Thursday, after the pope was greeted by Queen Elizabeth -- titular head of the Church of England founded when Henry VIII broke with Rome in 1534 -- he got to the heart of his message in his first speech on British soil as Roman Catholic leader.
He spoke of the "deep Christian roots that are present in every layer of British life."
Groups that plan to protest against the pope's trip, only the second in history, include atheists, secular organizations, and those who want the pope to be held legally responsible for the sexual abuse scandals.
The pope, out to win over one of Europe's most secular countries, reminded Britons to beware of 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 a 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.
The National Secular Society criticized the pope, saying his comments about British society were wrong.
"The secular identity of the British people is not something to criticize, but to celebrate. We have rejected dogmatic religion devoid of compassion," it said in a statement, adding that the Church discriminates against gays and women.
Later, at an open-air mass in nearby Glasgow, during which TV show "Britain's Got Talent" singing sensation Susan Boyle sang, the pope told the 65,000 people attending that followers should not be afraid to promote their faith.
"There are some who now seek to exclude religious belief from public discourse, to privatize it or even to paint it as a threat to equality and liberty," he said during his homily.
He is expected to return to the theme during a speech to civic leaders at Westminster Hall on Friday.
The German pope spoke glowingly of Britain's history and, significantly because of his own background, praised its people for standing up to the "Nazi tyranny" that was wreaked on the country by his own people in World War Two.
The Queen also spoke of the common Christian heritage that Anglicans and Catholics shared, and of their common belief that religion should never be allowed to justify violence and that dialogue could transcend "old suspicions."
Outside, around 150 protesters waved gay rainbow flags and banners saying "Pope opposition to condoms kills people" and "Stop protecting pedophile priests."
(Additional reporting by Anna MacSwan; Editing by Elizabeth Fullerton)