Four men have been arrested in north London under terrorism laws.
Officers raided three addresses in Barnet and Finchley and are quizzing the men at a London police station.
Armed police made the raids early on Friday as part of a long-running operation involving MI5, believed to be targeting an Iranian dissident group.
The men are not thought to be connected to al-Qaeda, nor planning terrorist attacks in the UK, said BBC home affairs correspondent Danny Shaw.
The four men arrested are aged 31, 37, 58 and 63, but their nationalities have not been revealed.
Officers, who carried out Friday's operation under section 41 of the Terrorism Act 2000, are searching the addresses that were raided.
Two of the men were arrested in a vehicle in High Road, Barnet.
They are all being questioned at London's high security Paddington Green police station.