Approximately 200 right-wing demonstrators—dressed in black, some wearing masks, shouting nationalist slogans and making Nazi salutes—interrupted a memorial service at a square in Brussels on Sunday, the Guardian reports. Riot police cleared the square with water cannon.

Those in the square had gathered to honor the 28 killed and 340 injured in last week’s suicide bombings at the airport and subway station, despite the fact that a planned March Against Fear was cancelled a day before out of concerns over another terror attack.

One witness, Adam Liston, told the BBC there had been a “really positive atmosphere” in the square.

“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).

The BBC originally reported that the group identified themselves as Fascists Against Terrorism, but that report was later revised to Casuals Against Terrorism.

According to Reuters, Mayeur said the group were “scoundrels.”