Belgian Prime Minister Charles Michel on Sunday called for restraint in the face of self-proclaimed "hooligans" who have gathered at a makeshift memorial site for the victims of last week's triple bombing attack in Brussels.
Around 450 black-clad football supporters - some hooded and apparently drunk - gathered at a square in central Brussels chanting "Belgian hooligans, we are at home," the Belga news agency reported.
Reported to have come from the town of Vilvoorde, north of Brussels, the protesters gathered at the square outside the former stock exchange in Brussels, despite calls by authorities not to hold any demonstrations in the wake of the attacks.
Around 10 people have so far been taken into custody, Belga reported. It added that police have begun dispersing the protesters.
The state broadcaster RTBF quoted witnesses as saying that some of the people present appear to be far-right activists.
One man at the scene described the atmosphere as "very aggressive." He added that the crowds dressed in black were facing off with a more colourful bloc of pro-diversity demonstrators.
"We are doing what is necessary with the police to send these protesters back home," said Michel. "At the same time, we also ask everybody to keep their cool and to stay calm so that the police can continue their work."
Calling for the respect for those mourning the loss of life in Tuesday's terrorist attacks, the premier condemned the impromptu demonstration, saying "it is completely inappropriate that protesters interrupt [the period of] contemplation at the Bourse. ... People are gathering to find comfort."
Earlier Sunday, police in Belgium carried out 13 terrorism-related raids in neighbourhoods of the Belgian capital and near the northern city of Antwerp, prosecutors said in a statement.
Nine people were taken into custody for questioning, with five of them released by the afternoon.
The raids were carried out in connection with a "terrorism file," the prosecutors added, without specifying which case.
Germany's Die Welt newspaper reported that European security authorities are searching for at least eight further suspects linked to attacks carried out by Islamic State-affiliated terrorists in Paris and Brussels.
The Islamists, mostly French and Belgian, are believed to be on the run in Syria or within Europe, the newspaper reported, citing security sources.
The terrorist attacks in Paris on November 13 left 130 people dead, while the most recent attacks on March 22 in Brussels left 31 people dead.
It was unclear whether the reported suspects were among those arrested in Belgium on Sunday.
Another man detained in the Brussels neighbourhood of Schaerbeek on Friday afternoon has been placed under arrest and charged with participation in the activities of a terrorist group, the Belga news agency quoted prosecutors as saying on Sunday.
The man, identified only as Abderamane A, was taken into custody in connection with the investigation that led to the arrest in France on Thursday of 34-year-old Reda Kriket, a man said to have been in the advanced stages of planning a new terrorist attack.