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.
A rally is being held at City Hall in Cardiff today as part of a campaign calling on Cardiff Council to honour their pledge to establish a Welsh-medium school in Grangetown.
Campaigners say access to Welsh-medium education is especially difficult for many in the Butetown, Riverside and Grangetown communities, with some children having to face round trips of up to 8 miles a day with no financial assistance for transport.
A firefighter has been hospitalised with burns after tackling a blaze at a Cardiff property housing a cannabis factory.
South Wales Fire & Rescue crews were called to the property in Grangetown at 2.48am, where they found the roof of the building alight.
A spokesperson said the fire had spread to the rear extension and was threatening neighbouring properties.
At the height of incident around 42 firefighters were at scene, as well as ambulance service workers and South Wales Police.