Counter terrorism detectives have arrested a suspect on a flight from Canada to Heathrow.
Officers swooped on a 28-year-old Canadian national at Heathrow Airport after he arrived on a flight from Canada just after midday on Monday.
A 56-year-old man was arrested in east London at 5.40am as part of the same operation.
They were being held under section 41 of the Terrorism Act 2000 at a west London police station.
Police searches of three addresses in east London were ongoing on Monday afternoon.
A police statement said: “The men are currently being held under section 41 of the Terrorism Act 2000, at a west London police station. Police searches of three addresses in east London are ongoing.”
Anyone with information is asked to call police on 101 or to remain anonymous Crimestoppers on 0800 555 111.