Five men are suspected of preparing a terrorist act near an English air base the US uses in its war against Iran.
Dozens of homes around Gloucestershire's RAF Fairford were evacuated and a bomb disposal robot examined three "suspicious" vans.
British police have not detailed the identities, nationalities and potential motives of the arrested men.
London's Metropolitan Police said authorities received a call about three vehicles travelling near to Gloucestershire's RAF Fairford at about 12:45am on Sunday, local time.
The arrests led to the evacuation of at least 85 nearby homes at about 7am.
Julian Tudsbery, who has lived in Whelford for 30 years, said another resident reported seeing three "suspicious vans" in the village early on Sunday.
The five men were at first arrested on suspicion of committing offences under the UK's Explosives Act in the Whelford area, which backs onto RAF Fairford and hosts the headquarters of the US Air Force's 501st Combat Support Wing and the 420th Air Base Squadron.
Suspects arrested under the Terrorism Act 2000 can be detained for up to 14 days without charge, unlike the 24-hour limit for standard arrests, without an extension application.
In March, officials said the number of national security cases involving activity by hostile states including Iran increased by 50 per cent in the six months to December 2025.
RAF Fairford has been used by American bombers for strictly limited strikes against Iranian missile sites under an agreement with former British Prime Minister Sir Keir Starmer, who voiced opposition to the war after US and Israel launched it on February 28.