In scenes that compounded a week of grief for Belgians, black-clad protesters shouting anti-immigrant slogans moved in on the makeshift memorial at Place de la Bourse where hundreds of people had gathered in a show of solidarity.
Under-fire Belgian authorities meanwhile detained four terror suspects after carrying out 13 raids as they seek to round up a web of jihadists with links to the carnage in the Belgian capital and to attacks and plots across the border in France.
"This is our home" and "The state, Daesh accomplice" around 300 hooligans chanted, using an alternate term for IS, as they gathered near the square by the stock exchange building, AFP journalists witnessed.
Some trampled on the carpet of flowers, candles and messages left at the site by mourners in recent days while at least one wore a mask with a well-known far-right symbol.
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Police urged the mourners, who included some Muslims, not to provoke the hooligans, but some chanted "Fascists! Fascists! We're not having it!"
Around 10 people were arrested, police told AFP.
Brussels mayor Yvan Mayeur said police had done "nothing" to stop the hooligans coming to Brussels despite having advance warning, adding that he was "appalled" that "such thugs have come to provoke residents at the site of their memorial."
Belgian Prime Minister Charles Michel said he "emphatically" condemned the demonstration.
The mourners gathered despite the fact that organisers had earlier called off a "March Against Fear" in Brussels today at the request of Belgian authorities, who said police needed the resources for the attacks investigation.