Belgian police fired water cannon to disperse a large group of demonstrators who stormed a central Brussels square.
They invaded the Place de la Bourse as people paid tribute at a makeshift memorial for victims of last Tuesday’s deadly attacks.
Riot police intervened to try to restore order after the group confronted Muslim women in the crowds, made Nazi salutes and chanted.
The attacks at Brussels airport and on the metro killed 28 people.
A “march against fear” planned for Sunday was called off after the authorities said it would strain police resources for the investigation.
Belgian police also carried out 13 new raids and took more people in for questioning on Sunday as part of investigations into the attacks.
A man already in Belgian custody was reported to have been charged in connection with a foiled attack in the Paris region.
The BBC’s Anna Holligan in Brussels says the group involved actually call themselves Casuals against Terrorism – not Fascists against Terrorism as earlier reported.
Some of the hundreds-strong group wore balaclavas and anonymous masks. Many wore black clothing. Some reports described them as football hooligans.
One of them, called Andres, told AFP news agency: “We are football hooligans, we don’t have anything to do with politics.”
“We are here for the victims and to pay our respects,” he said.
AFP said about 10 people were arrested.
One witness, Adam Liston, told the BBC there had been a “really positive atmosphere” in the square, as people crowded round the floral tributes in solidarity with the victims.
“Then a bunch of skinheads just turned up, marched into the square, and started a major confrontation with the peace protesters. They got in the face of the protesters and police. They set off flares and chanted and it was getting quite ugly,” he said.
He added that although it had been “pretty non-violent”, it looked like it “could have kicked off”.
The Belgian prime minister and the city mayor have strongly condemned the behaviour.
“I am appalled by what is happening, to learn that such thugs have come to provoke residents at the site of their memorial,” said Brussels mayor Yvan Mayeur.
PM Charles Michel said it was “highly inappropriate that protesters have disrupted the peaceful reflection at the Bourse (stock exchange).