Counter-terror officers have arrested a 55-year-old man at Gatwick Airport.
The suspect was held on suspicion of encouraging terrorism and dissemination of terrorist publications after flying into the UK from Morocco, police said.
Scotland Yard said he was arrested shortly after 11am on Saturday and is in custody at a police station in south London.
The Metropolitan Police said in a statement: "On Saturday April 7, detectives from the Met's counter-terrorism command arrested a 55-year-old man on suspicion of encouragement of terrorism and dissemination of terrorist publications, contrary to the Terrorism Act 2006."
- Press Association