A report on RTL info said the youngsters had firebombed and damaged the main police station in the Anderlecht area of Brussels, having spread the word by text message.
The crowds also broke car windows and smashed up phone boxes and the state is expected to flood the area with extra police all weekend.
Anger erupted over reports of police maltreatment of prisoners at the nearby Forest jail earlier this year, during a strike by the usual wardens. A prison watchdog has reporrted instances of torture, degrading treatment and islamophobic insults.
7s7 Belgique reported that groups of rioters had met up around the Place de la Vaillance and near the Aumale metro station, before splitting up and reforming elsewhere.