The arrests were made in the Schaerbeek district. There is no word yet on the identities of the suspects or their possible connection to the attacks.
Separately, in France, a suspect who was plotting an attack has been arrested near Paris, officials said.
The Brussels bombings have been linked to last November's Paris attacks.
So-called Islamic State (IS) has claimed the attacks in both Paris and Brussels.
The arrests in Schaerbeek were made late on Thursday, and followed house-to-house searches in the area.
Residents said they heard explosions during the police raids but the cause was unclear. Also on Thursday evening, French police launched an anti-terror operation in Argenteuil, north-west of Paris, following the arrest hours earlier of a man suspected of planning an attack.
Interior Minister Bernard Cazeneuve said the suspected militant, of French origin, was in an "advanced stage" of a plot, adding that no connection had been made to either the Brussels or the Paris attacks.
Last November, 130 people died after militants opened fire and detonated bombs in a number of locations in the French capital.
