A 19-year-old man has been arrested by counter terrorism police for spraying extreme right-wing graffiti ahead of a peace march.
Several far-right messages and Nazi symbols were left on buildings along the planned route of an anti-racism event in Cardiff.
Detectives from the Wales Extremism and Counter Terrorism Unit arrested a teenager from Cheltenham on Thursday on suspicion of conspiracy to incite racial hatred and conspiracy to commit arson.
Police say the graffiti and fly-posting suggested links to several known extreme right wing groups.
The vandalism, which included swastikas, the words Nazi Zone and offensive posters, were first spotted by parents during their school run in March this year.
Greg Pycroft, from the Grangetown area, was on his way home from dropping his sons at nursery when he noticed the symbols and words on buildings near Grange Gardens, a small park to the west of the city centre.
Mr Pycroft said in March: “I live close to the park and have lived in Grangetown for over 10 years and this is the first time I’ve seen this kind of graffiti and vandalism.
“The scale and brazenness of it was shocking, but its proximity to the local kids catching their bus to school this morning was the more upsetting, since a large number will be teenagers and they’ll know, or are beginning to understand the hatred behind that symbol.
“That was why I wanted it removed before home time this afternoon.”
Locations targeted with graffiti included the Senedd, home to the Welsh Assembly, the Millennium Centre and Cardiff University buildings.
South Wales Police said the 19-year-old suspect was brought into custody on Thursday and remains in custody.
Detective Superintendent Jim Hall of Wectu said: “Officers from Wectu worked with South Wales Police to investigate several instances of racially or religiously aggravated criminal damage in the Cardiff area between October 2017 and April 2018.
“The incidents appeared to support an extreme right wing ideology and suggested links to several known extreme right wing groups.
“Today’s arrest by Wectu will help to reassure communities that matters like these are taken very seriously and all possible lines of inquiry are fully investigated to identify and prosecute offenders.
“Extremism will not be tolerated within our communities and appropriate action has been taken to address any concerns and risks identified.”