Zadar Police Department spokeswoman Sandra Jurincic Poljak said the investigation was still under way and added that it could be extended to include more people.
About 50 people took part in yesterday's gathering, which was held in a hall displaying the photos of WW2 Ustasha leader Ante Pavelic and Ante Gotovina, the fugitive general wanted by the UN war crimes tribunal in The Hague.
After the gathering, about 30 participants walked through downtown Zadar carrying a photo of Pavelic, wearing black and displaying Ustasha insignia.
Zadar mayor Ana Lovrin condemned the incident and said she expected state bodies to act in line with the law.