The man, 23, fainted at a police station and then died later in hospital, according to Brussels prosecutor's office.
ADVERTISEMENTBelgium authorities announced on Wednesday that a probe is to be launched into the death of a black man detained by police.
The 23-year-old, identified only with the initial I.B., died in Brussels on Saturday.
The Brussels prosecutor's office said on Wednesday that an investigative judge would be appointed to lead the probe.
The announcement came as some 400 people, holding Black Lives Matter signs, demonstrated in central Brussels asking for authorities to shed light on the incident.
The peaceful demonstrations turned into clashes with a number of demonstrators breaking off from the main group. There was some destruction to public property, as well as damage to police vehicles.
Investigation
According to the prosecutors, I.B. was arrested after he allegedly tried to run away from officers checking a group of people gathered in central Brussels. They were there despite a ban on public gatherings imposed to curb the spread of COVID-19.
He was then taken to a police station, where he fainted, prosecutors said. He was transferred to hospital, where he was pronounced dead.
Local media reported that I.B. had started to record the police with his smartphone on Saturday when officers decided to carry out an ID check on him. The prosecutor's office said it has seized video surveillance images, both from the police station and at the scene of the man's arrest.
Belgium's Comité P, an independent body overseeing police services, is investigating and a coroner has been appointed to perform an autopsy as well as toxicology tests, the prosecutor office's.
It added that I.B's family and lawyer had met with the chief prosecutor on Wednesday morning and that they were given guarantees that "all means are and will be implemented to shed light on what happened".