Fifty people were arrested on Saturday in Brussels after police officers were attacked and shops damaged on the margins of a protest over the recent revelations of migrant slave markets in Libya, authorities said.
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."
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According to the RTBF public broadcaster, those arrested during the violence were aged between 15 and 18.
A police car was also targeted and projectiles were thrown, Van de Keere said. A large number of officers were deployed and calm was restored by the early evening.
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 for as little as $400.
This is the third time that violence has erupted in Brussels in a few weeks.
On 15 November, 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.