The Vatican said the pope was calm despite the pre-dawn arrests and planned no changes to his schedule.
Acting on a tip, police detained the men, aged 26 to 50, under the Terrorism Act at a cleaning depot in central London after receiving information about a possible threat. The men were being questioned at a London police station and have not been charged. Police said an initial search of that business and other properties did not uncover any hazardous items.
The pope's visit has divided opinion in officially Protestant, highly secular Britain. The trip has been overshadowed by disgust over the Catholic Church's clerical abuse scandal and opposition from secularists and those opposed to the pope's stances against homosexuality and using condoms to fight Aids.
The detained suspects worked for a contractor on behalf of Westminster Council, the authority responsible for much of central London. The pope will still address British politicians, businessmen and cultural leaders in Westminster Hall, part of the Houses of Parliament, later Friday.
The depot were the men were arrested is responsible for cleaning another part of London that the pope is not due to visit, police said. Police confirmed that some of the suspects were thought to be from outside Britain but declined to comment on media reports they were of Algerian origin.
Veolia Environmental Services, the cleaners' company, had no immediate comment on the arrests.
At the scene of the arrests in Chiltern Street, close to London's Madame Tussauds' tourist attraction, police cordoned off part of the road, removing items from the Veolia depot and examining nearby garbage cans.
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.
News of the arrests came as the pope was meeting representatives of other religions, including Muslims and Jews, and stressing the need for mutual respect, tolerance and freedom. The Vatican said the pope was informed of the arrests and was pleased he could stick to his schedule.
"We have complete trust in the police," Vatican spokesman Rev. Federico 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."
Hours after the arrests, Benedict met with the Archbishop of Canterbury Rowan Williams, the spiritual leader of the worldwide Anglican Communion, at his London residence. The meeting came amid new tensions following Benedict's unprecedented decision last year to make it easier for Anglicans opposed to the ordination of women bishops to convert to Catholicism.
Benedict and Williams greeted each other warmly. Benedict said flat-out he had no intention of speaking of difficulties "that are well known to everyone here". Rather, he stressed the need for Christians to work together and bring a greater sense of virtue into public discourse.
Williams, who has not hidden his dismay over the Vatican's invitation to conservative Angligans, also stressed the ongoing effort to bring the two churches back together, saying each side was "made less by the fact of our dividedness". He praised Benedict for his constant call to bring faith into public policy - a theme Benedict was to explore further Friday in his major address at Westminster Hall.
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.
There have been no major known attempts against Benedict during his five-year papacy, although he was knocked down at Christmas Eve Mass in 2009 by a mentally unstable woman who jumped the security barricade inside St. Peter's Basilica. In 2007, a man jumped the barricade in St. Peter's Square and grabbed the pope's vehicle before being subdued by guards.
Benedict's predecessor Pope John Paul II was wounded in an assassination attempt in 1981 in St. Peter's Square. Police in the Philippines also disrupted an alleged plot to assassinate John Paul in Manila in 1995.
Benedict was nearly 30 minutes late for his first event Friday morning; the Vatican attributed the delay at the time to logistical problems. It wasn't known if the arrests contributed to the delay.
The pope was then given a boisterous welcome by thousands of cheering Catholic schoolchildren at St. Mary's University College in London, where he urged young people to ignore the shallow temptations of today's "celebrity culture".
Benedict also told their teachers to make sure to provide the children with a trusting, safe environment - the second time in as many days that he has referred to the church sex abuse scandal.
On Thursday, the pope acknowledged that the Roman Catholic Church had failed to act quickly or decisively enough to remove pedophile priests from ministry, but US activists said much more needs to be done. Vatican officials haven't confirmed that Benedict will meet with abuse victims in Britain, but UK organizers say arrangements are being made.
"Our responsibility toward those entrusted to us for their Christian formation demands nothing less," Benedict said. "Indeed, the life of faith can only be effectively nurtured when the prevailing atmosphere is one of respectful and affectionate trust."
Polls in Britain indicate widespread dissatisfaction with the way Benedict has handled the sex abuse scandal, with Catholics nearly as critical of him as the rest of the population.
Outside the London university hall, some 4000 young students, outfitted in prim school uniforms and waving small white-and-yellow Holy See flags, serenaded the pontiff Friday with gospel hymns and songs.
The students, from England, Scotland and Wales, gave Benedict a tie-dyed stole and three books tracing the history of the Catholic Church in Great Britain. The 83-year-old Benedict appeared relaxed and happy, gently greeting children and kissing them on the head.
In a surprise move, Becky Gorrod, 39, who had been standing outside the gates of St. Mary's holding her 8-month-old daughter Alice, was ushered in to meet the pontiff as the crowd cheered.
"My husband's never going to believe me," Gorrod told journalists. "They opened the car door, and the pope got out. Then the (pacifier) fell out of Alice's mouth, and the pope bent down and picked it up! The pope! How mad is that?"
She said the pope then kissed Alice on the forehead.
A few blocks away, about 30 people protested, 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."
Michael Clark, 60, said he was protesting because he was gay and annoyed that the pope's visit was expected to cost British taxpayers Stg12 million ($NZ25.78 million) for security.
"That means it's being supported by taxpayers and people who may not have the same ideas," Clark said. "Sexuality is not evil."
Benedict began his four-day UK state visit on Thursday, greeted by Queen Elizabeth II at Holyroodhouse Palace in Edinburgh, Scotland. He wraps it up Sunday in Birmingham when he beatifies the 19th century Anglican convert Cardinal John Henry Newman.
Catholics are a minority in Britain at 10 percent, and up until the early 19th century they endured harsh persecution and discrimination and were even killed for their faith. King Henry VIII broke with Rome in the 16th century after he was denied a marriage annulment.
