Counter terrorism officers stopped a man at Stansted Airport on Sunday night.
He was arrested on Monday on suspicion of terrorism offences and has been taken to a London police station where he remains in custody.
The 21-year-old had returned to the UK from Istanbul, Turkey, via Stuttgart.
Detectives says the arrest was not in response to any immediate threat or risk. Detectives have carried out searches of three addresses in connection with this investigation.
More top news
Chilly, with clear skies overnight. Sunny Saturday, and warm inland.
Three men caught on CCTV assaulting a man in the violence following last year’s Luton Carnival have been given jail time.
We look ahead to a big weekend in League Two as the battle for the play-offs intensifies.