Anti-terrorism officers have arrested a man on suspicion of supporting a proscribed organisation, the South East Counter Terrorism Unit (Sectu) said.
The suspect, 23, was held this morning under section 12 of the Terrorism Act 2000 in Rochester, Kent, by Sectu officers working with Kent Police colleagues.
He is being questioned at a police station in the Kent area following what officers said was a "pre-planned" operation not linked to any immediate threat in the UK.
Officers said the arrest related to an investigation by Sectu and Kent Police.
Detective Chief Inspector Keith Roberts, of Kent Police, said: "I would like to reassure the community that this investigation should have very little impact on local residents.
"The arrest was pre-planned and was not made in response to any immediate threat to public safety. Our officers will continue to maintain a presence in the area, offering reassurance to the local community."