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Police personnel line the streets surrounding the British Houses of Parliament in London on Friday ahead of a visit by Pope Benedict XVI.
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Pope Benedict XVI greets schoolchildren during a Friday gathering at St. Mary's University College in Twickenham, England.
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London authorities were questioning six men arrested Friday over a possible threat to Pope Benedict XVI during his four-day state visit to Britain.
Scotland Yard acted on a tip-off received overnight, detaining five suspects after sending in counterterrorism officers to raid a cleaning depot in central London, authorities said. Police said a sixth suspect was arrested at his home later in the day, but gave no further details.
It was not immediately clear if a suspected terrorist plot involved an attack on the pope himself or venues where he was scheduled to appear.
The suspects arrested in the raid, ages 26 to 50, worked for a contractor charged with cleaning on behalf of Westminster Council, the authority responsible for most of central London. Police confirmed that the men were thought to be from outside Britain, but declined to comment on news reports that some or all of them are of Algerian origin.
Veolia Environmental Services, the cleaners' company, had no immediate comment on the arrests.
Initial searches uncovered no hazardous items, police said, adding that searches were under way at premises in north and west London. Officials said the suspects were being questioned but had not been charged.
"Following today's arrests, the policing arrangements for the papal visit were reviewed, and we are satisfied that our current policing plan remains appropriate," police said in a statement.
The Vatican spokesman, the Rev. Federico Lombardi, said the pope had been informed of the arrests, but that Benedict was not worried and planned to maintain his schedule.
"We have complete trust in the police," Lombardi told reporters. "The police are taking the necessary measures. The situation is not particularly dangerous. The pope is happy about this trip and is calm."
The pope's security on this trip has been visibly higher than on previous foreign trips, and Vatican officials have acknowledged that Britain represents a higher security threat than the other European countries Benedict has visited this year, including Portugal, Malta and Cyprus.
Benedict travels with his own security detail, headed by chief papal bodyguard Domenico Giani. Benedict's white, bulletproof Popemobile is flanked by eight to 10 dark-suited bodyguards who jog alongside, scanning crowds for potential threats as the pope waves to well-wishers from inside.
There have been no major known attempts against Benedict; his predecessor Pope John Paul II survived an assassination attempt in 1981.
News of the arrests came on what was otherwise an upbeat day for the pontiff. Benedict began the day among several thousand Catholic schoolchildren, who sang, prayed and cheered as the pope presided over a gathering called the Big Assembly at St. Mary's University College in southwest London.
The pope, clad in his white robes, spoke of the importance of providing a safe, trusting, respectful environment for children. The 83-year-old pontiff appeared relaxed, gently greeting each child and kissing each on the head.
"For us, our school, it's very important," student Maresha Barnes, 13, said. "We have a picture of the pope in the lunch hall."
Benedict told the children they should work to become saints and not be swayed by the materialistic goals of wealth and fame prevalent in today's "celebrity culture."
A few blocks away, some 30 people opposed to the pope's stand against homosexuality and the church's ban on using condoms to fight AIDS held a protest, holding up inflated condoms and posters. "Condoms are not crimes," read one. Another read: "Science flies you to the moon; religion flies you into buildings."
Later in the day, the pope met with representatives of other faiths before visiting the Archbishop of Canterbury Rowan Williams at the Anglican leader's residence in London. Benedict angered Williams last year when he suggested that disaffected Anglicans should become Catholic.
The two religious leaders greeted each other warmly, with Benedict saying he had no intention to bring up past differences and that instead Christians should work together.
Williams agreed that each side was "made less by the fact of our dividedness."
The archbishop praised Benedict for his constant call to bring faith into public policy, a theme the pontiff touched on in a speech in Westminster Hall attended by former prime ministers Margaret Thatcher, John Major, Gordon Brown and Tony Blair, who recently converted to Catholicism.
Benedict said there were "worrying signs" that religion was being excluded from its "legitimate role … in the public square."
"There are those who would advocate that the voice of religion be silenced, or at least relegated to the purely private sphere," he said. "There are those who argue that the public celebration of festivals such as Christmas should be discouraged, in the questionable belief that it might somehow offend those of other religions or none."
The talk in Westminster Hall was drenched with symbolism. For 900 years, the structure has stood at the heart of power in England, forming the stage for many scenes in the nation's long conflict with Catholicism. In this hall, St. Thomas More was sentenced to death for refusing to recognize King Henry VIII as the head of the Church of England after the monarch split from Rome.
Catholics are a minority in Britain, and up until the early 19th century, they endured harsh discrimination and were even killed for their faith. Henry broke with Rome in the 16th century after the Vatican refused to annul the first of his many marriages.
Benedict's controversial visit in largely secular Britain has drawn fire from both Protestants and humanist groups. Many who oppose his visit believe that the Roman Catholic Church has not done enough to address a global scandal involving pedophile priests.
Polls in Britain indicate widespread dissatisfaction with the way Benedict has handled the abuse scandal.
En route from Rome to London on Thursday, the pope made his strongest statement to date on the scandal, acknowledging that the Vatican had not acted quickly or decisively enough when the scope of the problem -- uncovered decades ago -- became apparent.
NPR's Philip Reeves reported from London for this story, which contains material from The Associated Press.