A 36-year-old man has been arrested on suspicion of preparing terrorist acts, police said.
Counter-terrorism detectives swooped as he arrived at London's Heathrow Airport on Thursday, Scotland Yard said.
He is currently being held at a west London police station.
The terror threat level in the UK is currently "severe", which means an attack is highly likely. It is the second highest level, below critical.
In a statement the Metropolitan Police said: "The 36-year-old man was arrested on suspicion of preparation of terrorist acts (contrary section 5 Terrorism Act 2006).
"He has been taken into custody, under the Police and Criminal Evidence Act (PACE), at a west London police station where he remains at this time.
"Enquiries by detectives continue."