Pope Benedict XVI has begun his state visit to the UK with an admission that the Catholic church was too slow to tackle paedophilia by priests, and an attack on "aggressive secularism" and "atheist extremism".

Speaking to invited guests after his first meeting, with the Queen and the Duke of Edinburgh, the pope wasted no time in appealing for a role for religion in society, while laying into what he termed "the more aggressive forms of secularism". And in what appeared to be a swipe at some of his most vociferous critics, he linked "atheist extremism" to the Nazism that Britain had fought in the second world war.

On the flight from Rome, the pope used his strongest language to date on the church's involvement in sex abuse, saying he deplored its failure to act swiftly and decisively in the past and that the Catholic church was "at a moment of penitence" over its record on clerical sex abuse.

The papal flight touched down at Edinburgh airport just after 10.15am, where the pope was met by Prince Philip.

Sex abuse scandal

The pope told reporters on board the plane that paedophilia was an "illness" whose sufferers had lost their free will.

Answering previously submitted questions during a 15-minute briefing, he said: "It is difficult to understand how this perversion of the priestly mission was possible". He said he had learnt of the recent cases with sadness: "Sadness also that the church authorities were not sufficiently vigilant and insufficiently speedy and decisive in taking the necessary measures".

He said the first priority was to help the victims recover from the trauma they had undergone "and rediscover too their faith in the message of Christ".

Priests at risk of sexually abusing the young should be "excluded from all possibility of access to young people because we know that this is an illness and free will does not work when there is this sickness".

"We must protect these people against themselves."

The 83-year-old pope, who held his weekly audience the day before leaving for Britain, spoke with a noticeably hoarse voice in some of his replies.

A Vatican source said this was not the first time the pope had referred to paedophilia as a sickness, but that in the past he had more often described it as a sin or a crime.

It is expected the pope will meet victims during his tour of Britain, as he has done on visits to the US, Australia and Malta. The Vatican is maintaining as strictly confidential the identity of the people the pope will meet as well as the place and time of the encounter.

The pope praised British bishops for the way they had tackled the problem of clerical sex abuse.

'Aggressive secularism'

In a speech at the Palace of Holyroodhouse, the pope also praised Britain for its role in fighting Nazi Germany and forging the postwar consensus, but warned again of the dangers of what he termed "aggressive secularism".

The pope said that even in his own lifetime, "Britain and her leaders stood against a Nazi tyranny that wished to eradicate God from society and denied our common humanity to many, especially the Jews, who were thought unfit to live".

Driving home a point that is expected to be central to his four-day visit, Benedict went on: "As we reflect on the sobering lessons of the atheist extremism of the 20th century, let us never forget how the exclusion of God, religion and virtue from public life leads ultimately to a truncated vision of man and of society and thus to a 'reductive vision of the person and his destiny'."

The quote was from his own encyclical on social and economic issues, Caritas in Veritate, published last year.

In what might be regarded as a less than warm endorsement, the pope noted that the UK strove 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. Let it not obscure the Christian foundation that underpins its freedoms."

His choice of words echoes controversial comments made yesterday by a senior Vatican adviser who claimed Britain discriminated against Christians, and likened arriving in multicultural Britain to visiting "a third-world country".

The pope called on the British media to remember its global power and behave responsibly.

"Because their opinions reach such a wide audience the British media have a graver responsibility than most and a greater opportunity to promote the peace of nations."

He urged Britons to "continue to operate by the values of Cardinal [John Henry] Newman of respect, honesty and fair-mindedness". He was referring to the 19th century clergyman who will be beatified in a ceremony on Sunday.

Controversies

The visit itself has been criticised by a number of groups, including gay and women's rights organisations, and demonstrations are expected.

However the pope brushed aside the controversies that have preceded his arrival, saying he had also faced anti-clericalism and anti-Catholicism on his visits to France and the Czech Republic, where he had also had a "warm welcome" from the Catholic community.

"Naturally, Great Britain has its own tradition of anti-Catholicism. That's obvious. But it's also a country with a great history of tolerance," he said.

This afternoon the pope will celebrate mass at Bellahouston Park in Glasgow before flying to London tonight.

The papal trip will include a meeting with the prime minister, David Cameron, and a prayer vigil in Hyde Park, culminating in Newman's beatification ceremony in Birmingham on Sunday.

Up to 100,000 people were expected to line the streets of the Scottish capital as the pope was driven in the "popemobile" to the home of the leader of the Catholic church in Scotland, Cardinal Keith O'Brien.

A state car will later be used to take him to Glasgow's Bellahouston Park, where about 65,000 pilgrims are expected to attend a mass, which will be preceded by a musical performance from Britain's Got Talent star Susan Boyle.