A 21-year-old man has been arrested in east London on suspicion of terrorist offences.
Scotland Yard said he was held by counter-terror police in a dawn raid at an address in Hackney.
He was arrested on suspicion of assisting another to commit acts of terrorism, or to engage in any conduct for the committing of such acts, by facilitating another to travel abroad.
The arrest is understood to be linked to Syria, the war-torn state where hundreds of would-be British jihadis have travelled to fight alongside extremists.
Police are searching the address in Hackney where he was arrested.