An 18-year-old man from Grangetown in Cardiff has been arrested on suspicion of terrorism offences.
South Wales Police say he has been arrested under the Section 5, Terrorism Act 2006 for assisting in the preparation of an act of terrorism.
Police said that today's arrest was not linked to any immediate threat to public safety.
More top news
Showers this evening with heavier and more persistent rain arriving overnight. A wet start tomorrow but improving during the afternoon.
A mother from Neath has told how her teenage son collapsed and was left unconscious for five hours after taking a so-called legal high.
Two brothers from Cardiff are campaigning for their cousin's murder to be re-investigated, 76 years after she died.