Gloucestershire police declared a major incident at the Royal Air Force base, which is used by US forces, on Sunday morning. Several men were taken into custody on suspicion of offences under the Explosives Act. The incident has since been contained, police said.
Army bomb disposal experts are currently examining a number of vehicles near the base. About 30 emergency vehicles, including police cars, ambulances and fire engines, were stationed in the village of Fairford. Armed police blocked access to the main gate of the base.
"A number of properties are currently being evacuated following the declaration of a major incident," Gloucestershire police said in a statement. "We are working closely with our partners and are enacting the well-rehearsed plans we have in place." Evacuated residents were being taken to a nearby leisure centre.
The security scare comes after the British government gave authorisation to the US to use RAF Fairford to carry out strikes on Iranian missile sites that are attacking ships in the Strait of Hormuz. The base has been used to deploy US bombers since the conflict in Iran began on February 28.
On Saturday, US President Donald Trump rejected an Iranian plan to reopen the Strait of Hormuz within seven days, while threatening the possibility of further bombing.