Tensions soared in Bosnia's Serb republic after police arrested a man who accused top officials of covering up his son's killing for political reasons.
Davor Dragicevic was arrested on Tuesday after he failed to show up for questioning, police said.
His detaining sparked demonstrations in Banja Luka on Wednesday night as police were deployed in riot gear to contain protesters.
After several hundred people gathered at a park near the city's central square, they were moved on, before being reassembled outside a Serbian Orthodox church.
His "Justice for David" movement has demanded information about the death of his 21-year-old son David in March.
It follows demonstrations on Tuesday, where activists gathered in the central square of the city, demanding the truth about the death of Dragicevic's son.
Along with Dragicevic, authorities detained his wife briefly and other protesters, including opposition politicians.
Dragicevic claims his son, 21, who was found dead in March in a shallow river in Banja Luka, was captured, tortured and murdered.
Prosecutors investigating the case did not find anyone guilty of murder.
The European Union delegation and the office of Bosnia's international peace overseer expressed concerns about the arrests and urged all sides to refrain from violence.
"We have asked the Republika Srpska Ministry of Interior for an immediate explanation of the ongoing arrests of different persons associated with the 'Justice for David' movement," they said in a joint statement.
"We are following the developments closely in Banja Luka and urge everyone to stay calm and refrain from violence."
The association of Bosnia's journalists and the Transparency International corruption watchdog also condemned the arrest, saying they represented "political persecution" by the regime.
In Sarajevo, where another father is demanding the truth about his son's murder, a protest in solidarity for Dragicevic was scheduled for the evening.
Sources said police were ordered to clear the square of activists and prepare it for public New Year celebrations which Dragicevic had threatened to obstruct.
The government, controlled by Bosnian Serb leader Milorad Dodik, has rejected Dragicevic's allegations and urged him to stay out of the country's politics.
The republic has been semi-divided into Serb and Muslim-Croat areas since the breakup of Yugoslavia.