"Possible threat to the pope"

The BBC reported that the men had posed "a possible threat to the pope" but police refused to confirm or deny that. The Vatican said the trip would go ahead as planned and that the pope was calm.

The five unnamed men, aged between 26 and 50, were arrested around 5:45 a.m. (0445 GMT) on "suspicion of the commission, preparation or instigation of acts of terrorism," according to a police statement.

The pope has been heavily protected during his four-day visit to Britain, travelling in a custom-built bulletproof car surrounded by security officials.

Benedict has not been the target of any serious attacks but his predecessor was almost killed in an assassination attempt in 1981 and was the subject of several other attacks.

When the pope travels outside the Vatican is protected by the host country's police forces plus a small contingent of about a dozen Vatican security men.

The last terrorist attack in Britain was in July 2005, when four young British Islamists killed 52 people and wounded hundreds when they set off suicide bombs on three underground trains and a bus.

Protests

The Vatican was told of the arrests as the pope was arriving at a Catholic university in southwest London.

"We are totally confident in police and there are no plans to change the program," said Father Federico Lombardi. He said the pope was calm and looking forward to the rest of the visit.

At the university, Benedict reminded his church, reeling from evidence of widespread sexual abuse of children by priests, that its first priority was to provide a safe environment for children.

"Our responsibility towards those entrusted to us for their Christian formation demands nothing less," he told a gathering of Catholic schoolteachers and administrators.

In the first substantial protest of his delicate visit to Britain, several hundred people whistled and shouted "Pope must resign!" and "Shame!" as the papal motorcade entered a nearby Catholic school complex.

They held placards reading "Hypocrisy and lies" and "Catholic pedophile cover up."

The sex abuse scandal, in which priests who abused children were moved from parish to parish instead of turned over to police, has hounded Benedict's five-year-old papacy, even though most of the abuse took place decades ago.

On Thursday, the pope told reporters aboard the plane from Rome that he was shocked by what he called a "perversion" of the priesthood and acknowledged that the Church had not been sufficiently vigilant and decisive in dealing with the scandal.