Police were questioning a sixth man last night as counter-terrorism detectives continued to investigate a plot to attack the Pope.
Scotland Yard said the suspect, aged 29, was held by officers at a home in north London shortly before 2pm.
A spokesman said he was arrested under the Terrorism Act 2000 on suspicion of the commission, preparation or instigation of acts of terrorism.
Last night, police were searching eight homes in north and east London and two business premises in central London, including a street-cleaning depot.
Five street cleaners were arrested at gunpoint as armed officers swooped on their base as they prepared to start their shift shortly before 6am.
They work for Veolia Environmental Services, a contractor which employs 650 on-street staff to keep the streets of Westminster clean.
Searches of the depot and their homes in north and east London did not reveal any weapons or suspicious materials.
But senior detectives said they were compelled to act after a tip-off about suspicious activity late last night.
The security alert led to a review of the huge police operation surrounding the visit of Benedict XVI, but his itinerary went ahead as planned.
His spokesman said the Pope was informed of the arrests but remained "totally calm" and will continue to enjoy his trip.
The police operation began shortly before 6am at Veolia's Chiltern Street depot, in Marylebone, where staff are responsible for cleaning surrounding streets.
The five suspects, aged 26, 27, 36, 40 and 50, were held on suspicion of preparing an act of terrorism under the Terrorism Act 2000.
It is understood all five are of North African origin, including several from Algeria.
A huge security and public order operation swung into action yesterday as the Pope touched down in Britain.
Thousands of officers are involved in the operation from forces including the Met, Strathclyde, Lothian and Borders, West Midlands and British Transport Police.
The cost of policing the Pope's visit to Britain will exceed £1.5 million and is being coordinated by South Yorkshire Chief Constable Meredydd Hughes.
Senior officers said there was no information ahead of the visit to suggest any "specific group" wanted to attack the Pope.
But they warned people not to underestimate the "passion and the fervour" the visit would bring.
Police also interviewed mentally unstable people who they feared may pose a threat to the Pope.
A Metropolitan Police spokesman said: "Today's arrests were made after police received information. Following initial inquiries by detectives, a decision was made to arrest the five men.
"Following today's arrests the policing arrangements for the Papal visit were reviewed and we are satisfied that our current policing plan remains appropriate. The itinerary has not changed. There is no change to the UK threat level."
Leith Penny, of Westminster City Council, thanked police for their "swift" action as he said national security is of "critical importance".
He 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."
Father Federico Lombardi, a spokesman for the Vatican, said the Pope was informed of the threat during his visit to St Mary's University College, in Twickenham.
He was 30 minutes late for prayers in St Mary's Chapel, but it is not known if this was because of the unexpected intervention.
Father Lombardi said: "We are totally confident in the work of the police and Scotland Yard. We have no particular preoccupation with this.
"The police have already said that the information that they have collected until now demonstrates there is no need to change anything on the programme of the Pope and the security measures."
A spokesman for the Catholic Bishops' Conference of England and Wales said: "We have absolute confidence in the security operation to protect both the Pope and the public.
"The Pope is enjoying a wonderful visit, warmly welcomed wherever he goes."