Six men remain in custody after being held for questioning by anti-terrorism officers investigating a potential threat against the Pope's visit.
Searches conducted as part of the investigation have finished without the discovery of anything significant.
The Metropolitan Police says it searched both residential and commercial properties.
The six - who all work as street cleaners in Westminster - were arrested in the capital on Friday.
They are employed by Veolia Environment Services, a cleaning company contracted by Westminster Council.
At least five of the men are not British nationals. Most are thought to be Algerian.
The six - who are all arrested on suspicion of the commission, preparation or instigation of acts of terrorism - are aged between 26 and 50.
Armed officers arrested the first five at the company's Chiltern Street depot in west London as they were preparing to start their shifts. A sixth was later taken into custody.
In a statement from Scotland Yard, the Metropolitan Police said the men had been arrested in an operation under the Terrorism Act 2000 by officers from the force's Counter-Terrorism Command.
In the statement, the Metropolitan Police said policing arrangements for the papal visit were reviewed following the arrests, and that police were satisfied that the current policing plan for the Pope's visit remained appropriate.
"The itinerary has not changed. There is no change to the UK threat level," the statement added.
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.
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."
In all, the policing bill for the visit is expected to top £1m.