A sixth man was arrested on Thursday (September 21) in connection with the rush-hour bombing of a packed commuter train at Parsons Green station.
A 17-year-old male was arrested in Thornton Heath at around 12.05am after officers executed a warrant at an address.
Officers have begun a search of the property.
The latest arrest comes six days after 30 commuters were left injured following the blast on the District Line train.
To date, a total of six men have been arrested under section 41 of the Terrorism Act in connection with the Metropolitan Police's investigation.
An 18-year-old was arrested by Kent Police in the port of Dover at around 7.50am on Saturday (September 16) – a warrant for his further detention has been granted.
Later that day, a 21-year-old man was arrested in Hounslow at 11.50pm and a warrant for his further detention was granted at Westminster Magistrates' Court also.
In Newport, a 25-year-old man was arrested at around 7pm on Tuesday (September 19).
A 48-year-old and a 30-year-old were arrested the following day at around 5am at a second address in Newport.
All six are in custody at a south London police station.
Commander Dean Haydon, head of the Met Counter Terrorism Command, said the investigation continues to be “fast-moving” following the events on Friday (September 15).
“A significant amount of activity has taken place since the attack on Friday,” he said.
"We now have six males in custody and searches are continuing at five addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack.
"We anticipate that the searches will take some days to complete and may cause further disruption. However, it is important that we continue with these searches and I'd like to thank all those affected for their support, patience and cooperation."
The arrests in Newport were carried out by officers from the Met's Counter Terrorism Command, supported by colleagues from Gwent Police and the Welsh Extremism and Counter Terrorism Unit (WECTU).
Searches at the two addresses in Newport and the two addresses in Surrey are ongoing.
'Public vigilance remains as vital as ever'
Commander Haydon added: "As police and our partners do all we can to confront the threat, public vigilance remains as vital as ever.
"We urge the public to report any suspicious activity to the police by calling us, in confidence, on 0800 789 321, or in an emergency by dialling 999.
"We are asking the public to look out for anything that seems out of place, unusual or just doesn't seem to fit in with day-to-day life.
"It may be nothing, but if you see or hear anything that could be terrorist-related, trust your instincts, then act, and call us."
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