UPDATE 1-Pope's Security in Spotlight after London Arrests

* Not clear when attack was to have taken place

* Vatican says trip unchanged

* First substantial protest of trip to Britain

(Recasts with arrests during papal visit, Vatican statement)

By Philip Pullella and Matt Falloon

LONDON, Sept 17 (Reuters) - Pope Benedict's security was in the spotlight on Friday after London police arrested five men on suspicion of preparing an attack in Britain.

It was not clear if the planned attack was related to the pope's visit or when it was to have taken place, but it prompted police to take another look at security for the pontiff, who was on the second day of his visit to Britain.

The Vatican said the trip would go on unchanged. Police said they had reviewed their arrangements for the visit and concluded that their plan "remains appropriate".

The arrests were made by anti-terrorism police, who were searching a business in central London and homes in the north and east of the city but had not uncovered dangerous materials.

The five men, aged between 26 and 50, who were not named, were arrested around 5:45 a.m. (0445 GMT), police said in a 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.

Four young British Islamists killed 52 people and wounded hundreds when they set off suicide bombs on three underground trains and a bus on in July 2005.

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 programme," 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 paedophile 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. (additional reporting by Avril Ormsby and Maria Golovnina; Editing by Kevin Liffey) (For more on faith and ethics, see the Reuters religion blog FaithWorld at http://blogs.reuters.com/faithworld)