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
Powerboats and jet skis will be racing in Cardiff Bay later today in the fourth round of the SuperStock and AquaX UK Championships.
First Great Western rail workers are to continue a 72 hour strike in row over new trains
It should continue to become drier and brighter from the west. However, a sharp shower is possible.