Two 17-year-old boys from north-west London were detained in Istanbul along with a third male aged 19, the Met Police have said.
The three, who have not been named, were seized on Friday in the Turkish city but are now back in the UK where they are being held at a central London police station on suspicion of preparing terrorist acts, Scotland Yard said.
A Met police spokesman said: 'Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria.
'On Saturday, March 14 the three males returned to the UK and at approximately 11.10am were arrested on suspicion of preparation of terrorist acts.