Belgian authorities have held police raids in Brussels and two other Belgian cities and kept four of the people they picked up under detention.

The federal prosecutor's office says the raids are linked a "federal case regarding terrorism" but did not specify whether it had anything to do with the March 22 suicide bombing attacks in Brussels that killed 31 people and left 270 wounded.

The 13 raids in Brussels and the northern cities of Mechelen and Duffel came early on Sunday. An investigating judge will decide later Sunday whether the four new suspects will remain in custody. Five others were released after questioning.

Meanwhile, a Brussels square that has been a memorial site turned agitated on Sunday when black-clad men started shouting slogans and carrying a banner with an expletive against the Islamic State group.

Hundreds of people were remembering the 31 victims of Tuesday's attacks when dozens of men, some in balaclavas and anonymous masks, barged to the centre of Place de la Bourse and mounted the steps of the stock exchange building.

Riot squads joined plainclothes policemen to move the protesters away from the square. A water cannon sprayed the protesters.

The anti-IS rally came despite government appeals not to hold a march against fear Sunday in Brussels since the security forces were stretched too thinly to provide security.