The judge ordered the county prosecutor to file a request for investigation with the court within the next 24 hours or the man, identified as Zoran O., would be released.
Zoran O. and two other young men from Borovo, all ethnic Serbs, were arrested on Thursday morning after they had a verbal and then physical argument with HSP members and sympathisers on the terrace of the party's club in Vukovar's Borovo Naselje district on Wednesday evening.
Soon after the three men left the terrace, an unidentified person threw a hand grenade outside the club, slightly injuring two HSP members.
After a two-day criminal inquiry, the police brought the suspect Zoran O. to the investigating judge on Saturday morning, while the two other suspects were released on Friday evening.
After being heard by the investigating judge, Zoran O. was transferred to the District Prison in Osijek. If found guilty of the crime of endangering another person or property, he faces a prison sentence of one to eight years.