A group of people broke off from the demonstration against slavery in Libya before putting on masks and turning violent.
The incidents occurred in the busy commercial neighbourhood around Place Louise near the Belgian capital's city centre, police spokesman Ilse Van de Keere told AFP.
Brussels police posted on Twitter: "50 people (were) arrested and (there are) no injuries to be noted."
According to the RTBF public broadcaster, those arrested during the violence were aged between 15 and 18.
More From This Section
The US network CNN triggered a wave of condemnation when it aired footage last week of an apparent auction where black men were presented to North African buyers as potential farmhands and sold.
This is the third time that violence erupts in Brussels in a few weeks.
On November 15, an unauthorised rally organised by a young social media star escalated when the police tried to disperse the crowd.
Four days earlier, there was violence after Morocco's qualification for the 2018 football World Cup.