A 21-year-old man has been arrested on suspicion of being involved in the commission, preparation and instigation of terrorism offences.
The man was stopped at Stansted Airport at 8.40pm on Sunday after returning to the UK from Istanbul in Turkey, via Stuttgart in Germany, Scotland Yard said.
He was arrested by officers from the Metropolitan Police's Counter Terrorism Command at 1.10am today and taken to a London police station where he remains in custody.
This arrest was not in response to any immediate threat or risk, Scotland Yard said.
Detectives have carried out searches of three addresses in connection with the investigation, a Metropolitan police spokesman said.
"Inquiries are ongoing and we are not able to give further detail at present," he added.
A Scotland Yard spokesman could not confirm whether the man arrested was suspected of travelling to Syria.