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
Not too bad out there at the moment, with the heaviest of any showers easing during the first part of the night.
Sea Watch Foundation says it is investigating a reported but unconfirmed sighting of a killer whale off Ceredigion.
A man had to be freed this morning after an empty van rolled down the road and overturned, trapping him underneath.