Pope Benedict XVI arrived in Britain today for a four-day tour.

As he delivered a speech outside Holyrood House in Edinburgh after meeting the Queen, he warned the UK to beware of “aggressive forms of secularism” and urged people to retain traditional values while becoming a modern and multi-cultural society.

He said Hitler’s “atheist extremism” had led to the horrors of the Holocaust and praised Britain’s stand against it.

He added: “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.”

The Pope will hold an open-air Mass at Bellahouston Park in Glasgow tonight, with similar events in London on Saturday and Birmingham on Sunday.

Today, the 83-year-old made his strongest statement yet on the child sex abuse scandal in the Catholic Church, saying he felt “sadness also that the church authority was not sufficiently vigilant and not sufficiently quick and decisive to take the necessary measures”.

He said abusers suffered from an “illness” and that his priority was healing victims.

Responding to anger from some sections of the public about his state-funded visit, the Pope said Britain has a “great history of anti-Catholicism. But it is also a country with a great history of tolerance”.

Thousands of people lined the streets in Edinburgh today to see the Pontiff and police said there were few protests. However, hundreds of people are expected to join a march through London and a rally outside Downing Street against the visit on Saturday.

The rally will include speakers such as Peter Tatchell and Richard Dawkins, who have attacked the Pope for his condemnation of gay rights, contraception and abortion.

Mr Tatchell accused the Pope of covering up the “homosexuality” of 19th-century Cardinal John Henry Newman, who will be beatified during Sunday’s Mass. He claimed there was evidence to show that the cardinal and Father Ambrose St John were in a platonic gay relationship.

The Pope was also attacked today by National AIDS Trust and the Family Planning Association.

In a joint open letter, the charities said that his “irresponsible and discriminatory statements” on sexuality and sexual health were putting lives at risk.

They claimed that women were being killed and injured because they could not seek legal abortions in some countries, that condoms are crucial in the fight against HIV and that he was using his position to “incite and legitimise intolerance, prejudice and discrimination against lesbian, gay and bisexual people across the world”.