A sixth man has been arrested in London by the 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 the 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 the 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 the Westminster Council.
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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 the 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."