A sixth man has been arrested in London by police in relation to a potential threat to Pope Benedict XVI's visit.
His arrest, at 1345 BST, came after five men were seized at 0545 BST after counter-terrorism officers received intelligence of a potential threat.
All six, who were street cleaners, have been taken to a London police station.
The BBC's Danny Shaw said the arrests were carried out as a precaution. Police are searching a number of premises.
At least five of the men were not British nationals.
The cleaners worked for Veolia Environment Services, a major contract cleaning company that works for Westminster Council.
Armed officers arrested the first five men at the company's Chiltern Street depot, Paddington, as they were preparing to go on shift.
In a statement from Scotland Yard, the Metropolitan Police said that the men had been arrested in a Terrorism Act 2000 operation, launched by officers from the force's Counter-Terrorism Command.
All six men were arrested on suspicion of the commission, preparation or instigation of acts of terrorism.
They are 26, 27, 29, 36, 40 and 50 years old and most are understood to be Algerian. Police are continuing to search eight residential premises in north and east London and two business premises in central London.
Officers have not found any hazardous items.
In the statement, the Metropolitan Police said: "Today's arrests were made after police received information following initial inquiries by detectives.
"Following today's arrests policing arrangements for the papal visit were reviewed and we are satisfied our current policing plan remains appropriate. The itinerary has not changed. There is no change to the UK threat level."
The current official threat level in the UK is "severe", which means that security chiefs believe a terror attack is "highly likely".
The BBC understands that the information acted on by the police was received by Scotland Yard and did not involve intelligence gathered by MI5, the domestic security service.
Andrew Redhead, former national police firearms tactical adviser, told the BBC that "every allegation, every bit of information that comes to light will be vetted and taken seriously".
He added: "Obviously it would appear that the concerns are such [in this case] that persons have been arrested, and it is being taken seriously.
"Is this just a group of people speaking loudly and someone has overheard, or is it something more sinister? Obviously the police authorities and the specialists will have to work through that in due course."
Reacting to the first five arrests, Father Federico Lombardi, the Pope's press spokesman, said the Vatican was "totally confident" in Scotland Yard and the ability of its officers.
The papal team had no direct information about the police operation, he said, adding that the Pope remained calm and had been welcomed warmly everywhere he had gone.
"The police have already said that the information demonstrated that there is no need to change the programme," he said.
In a statement, Dr Leith Penny of Westminster City Council, said: "Veolia and Westminster City Council work closely with the relevant authorities to constantly ensure that all the people working on their behalf are subject to right to work checks as prescribed by the Home Office to assess their eligibility to work in the country.
"We are confident that these checks are robust and we will continue to work with the police and other authorities during this investigation."
The UK's top police officers from England and Scotland spent months planning the security arrangements for Pope Benedict XVI's visit to the UK.
Those plans included threat assessments, standard arrangements covering the safe transport of significant public figures and the potential need to control crowds during the visit's major events.
In all, the policing bill for the visit is expected to top £1m.
The BBC's Emily Buchanan, who was with the Pope at his visit to an assembly of children at a Catholic college in west London, described the security around the Pope as "extremely high... with no lapses".