As a sex abuse scandal rocked the Roman Catholic Church, what did Pope Benedict XVI -- then a cardinal and Vatican official -- know, and when? Watch the investigation "What the Pope Knew," September 25 & 26 at 8 p.m. ET on CNN U.S. and on September 25 at 7 p.m. CET and September 26 at 8 a.m. HK on CNN International.
London, England (CNN) -- Investigators in Britain arrested six men on suspicion of terrorism Friday and reviewed security arrangements for Pope Benedict XVI's trip there, authorities said.
Some news reports said the arrests involved a potential threat to the pope, who was in London on Friday, but the Metropolitan Police declined to say whether the case was directly linked to the pontiff's visit.
Police said they were satisfied the pope's security plan remains "appropriate," and the pope's itinerary did not change.
Vatican spokesman Father Federico Lombardi said the pope and his representatives were not particularly worried about the arrests.
"The police have already said that the information they have until now collected demonstrated there is no need to change anything about the program of the pope and the security," Lombardi told reporters in London.
He said he believed the police had simply taken "normal precautionary measures," and everyone remained calm.
"The pope is happy with the trip until now, and we can go on with the same joy as until now," Lombardi said.
Five suspects arrested earlier in the day are street cleaners in Westminster, the borough of London that includes Parliament, Westminster Abbey, and many of the city's tourist attractions, the Westminster City Council said.
They work for one of Westminster's contractors, Veolia Environmental Services, the council said.
It said all staff are subject to checks to make sure they are eligible to work in England, referencing questions about the men's nationality.
The men arrested appeared to be Algerian, a high-ranking source familiar with the investigation said, adding that some or all of them were probably in the country illegally.
The men were arrested around 5:45 a.m. at a business on suspicion of the commission, preparation or instigation of acts of terrorism, police said.
The men are ages 26, 27, 36, 40 and 50, and were taken to a central London police station to be interviewed by detectives.
A sixth man, 29, was arrested later in the day by counterterrorism detectives investigating the possible plot against the pope, Scotland Yard said.
The men were arrested under the Terrorism Act 2000, which allows police to hold them without charge for 28 days.
Searches were being carried out at a business in central London and at residential locations in north and east London, police said.
Searches of the men's homes have not turned up any terrorism-related paraphernalia such as bomb-making materials, the source said.
"Today's arrests were made after police received information," police said. "Following initial inquiries by detectives a decision was made to arrest the five men."
Police added there was no change to the threat level in the United Kingdom as a result of the arrests.
The arrests came on the second day of the pope's four-day trip to Britain, and on his first full day in the capital.
After holding an event in south London with Catholic schoolchildren and educators, then meeting with religious leaders, the pope met the archbishop of Canterbury -- the leader of the Anglican Church -- at his London residence of Lambeth Palace. It is the first time a pope has ever visited Lambeth Palace, and the visit marked a bridge between the two faiths.
"Part of what's at stake here is history," said John Allen, CNN's senior vatican analyst. "This is a schism between Roman Catholicism and the Anglican Communion that dates to the English reformation in the 16th century."
Pope Benedict and Archbishop Rowan Williams are friends, Allen said, but they have long-standing problems to deal with in the relationship between their churches. The Anglican Church has adopted fairly liberal policies on things like female priests, female bishops, and blessing gay marriages, much to the disapproval of the Catholic Church.
Earlier this year, many Anglicans objected when Benedict created new structures to welcome disaffected Anglicans into the Catholic fold.
"It is not my intention today to speak of the difficulties that the ecumenical path has encountered and continues to encounter," the pope said at Lambeth Palace, acknowledging the problems. "Rather, I wish to join you in giving thanks for the deep friendship that has grown between us and for the remarkable progress that has been made in so many areas of dialogue."
Stressing, however, the firm stance of the Vatican, the pope then said, "We recognize that the church is called to be inclusive, yet never at the expense of Christian truth."
The pope also spoke to members of the British Parliament at London's historic Westminster Hall.
It was there in 1535 that Thomas More, a Catholic, was convicted of treason and sentenced to death for refusing to accept King Henry VIII's marriage annulment and repudiate the pope after Henry broke with the Vatican and created the Anglican Church. The pope paused at the spot where More was condemned.
The pope told the legislators that the central questions in More's trial were still presenting themselves in "ever-changing terms as new social conditions emerge."
"I would suggest that the world of reason and the world of faith -- the world of secular rationality and the world of religious belief -- need one another and should not be afraid to enter into a profound and ongoing dialogue, for the good of our civilization," he said. "Religion, in other words, is not a problem for legislators to solve, but a vital contributor to the national conversation."
Later, at nearby Westminster Abbey, an Anglican church, the pope prayed alongside the archbishop of Canterbury at the tomb of Edward the Confessor, the English king who built the abbey and was buried there after his death in 1066.
He spoke once again about the commitment to unity among Christian churches while noting the obstacles.
"We know that the friendships we have forged, the dialogue which we have begun and the hope which guides us will provide strength and direction as we persevere on our common journey," he said. "At the same time, with evangelical realism, we must also recognize the challenges which confront us, not only along the path of Christian unity, but also in our task of proclaiming Christ in our day."
Thursday, Benedict said the Roman Catholic Church has not been vigilant enough or fast enough in responding to the problem of sexual abuse by priests.
"These revelations were for me a shock and a great sadness. It is difficult to understand how this perversion of the priestly ministry was possible," he told reporters aboard his plane to Scotland. "How a man who has done this and said this (in his priestly vows) can fall into this perversion is difficult to understand."
He added, "It is also a great sadness that the authorities of the church were not sufficiently vigilant and insufficiently quick and decisive in taking the necessary measures."
British people feel overwhelmingly that the pope has not done enough to punish priests who abuse children, according to a ComRes poll for CNN released as Benedict arrived in the country. Three out of four British people -- and two out of three Catholics in the country -- say he should do more to punish abusive clergy.
Thursday, tens of thousands turned out to hear the pope preach in Glasgow, Scotland, with members of the faithful describing it as a "happy day" and calling it a "great honor" that he had come.
Benedict started his visit in Edinburgh, where he held a meeting with the queen and greeted thousands who turned out to see him on the streets of the Scottish city.
Though a pope has visited Britain once before -- Pope John Paul II in 1982 -- this is considered the first state visit by a pope to Britain because it comes at the invitation of the queen, not the Catholic Church, as was the case 28 years ago.
CNN's Richard Greene, Laura Perez Maestro, Andy Carey and Antonia Mortensen in London, England, and Kathleen Johnston in Atlanta, Georgia, contributed to this report.