Three of the suspects were detained "outside the door of the federal prosecutor's office", in the city centre, spokesman Eric Van der Sypt said.
Two further people were arrested elsewhere in the city and the sixth was detained in Jette, on the outskirts of the capital, the spokesman said, without giving any further details on their identity.
"It will be decided tomorrow if an arrest warrant (charges) are brought against these people," he added.
At least two men identified by police from surveillance footage at the airport and metro station where the attacks took place are still at large.
Raids have also taken place in the Brussels district of Schaerbeek where the three airport attackers left from on Tuesday morning carrying three explosive-packed suitcases. There have been no arrests in the neighbourhood.