Anti-terrorism police, on high alert during a visit by Pope Benedict to the British capital, arrested six men on Friday on suspicion of preparing an attack.
Police moved quickly to make the pre-dawn arrests of five men who worked as street cleaners in the area in central London near parliament where the pontiff later spoke.
A sixth suspect was arrested about eight hours later but it was not clear if he worked for the same cleaning company contracted by the Westminster area of London.
Police, who searched eight homes and two businesses in the London, reviewed security arrangements after the arrests but decided they remained "appropriate".
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 pope visited the parliament area on Friday, where he met with the Archbishop of Canterbury and addressed British leaders.
Hundreds of protesters along the route called him the "anti-Christ" and shouted "shame" as they held up pictures of children who were sexually abused by priests in a scandal that has rocked the Roman Catholic Church worldwide.
Similar protests were held at a Catholic university the pope visited on Friday morning.
The six unnamed men, aged between 26 and 50, were arrested on "suspicion of the commission, preparation or instigation of acts of terrorism," according to police statements.
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 he 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 by setting off suicide bombs on three underground trains and a bus.
"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.
Several hundred protesters shouted slogans against the pope as he entered Lambeth Palace for talks with Archbishop of Canterbury Rowan Williams, spiritual head of the 80 million strong worldwide Anglican communion.
Williams and the pope, whose Churches split in 1534 and are now divided over issues such as women priests and gay bishops, both spoke of the importance of faith in society and agreed that Christianity should not be seen as a threat to freedom.
Later the pope told British leaders, including four former prime ministers, that religion had to be a "vital contributor" to national debate on a host of issues.
During the trip, the pope has asked Britons to beware of what he has called "aggressive secularism" that seeks to marginalise the views of believers.
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.
