The arrests were made during raids in central Brussels, Jette and the Schaerbeek neighbourhood -- where police found a large stash of explosives and other bomb-making material.
Brussels is still reeling from IS suicide bombings at the city's airport and a metro station in March which killed 32 people.
Ibrahim El Bakraoui, who helped kill 14 people when he set off his suicide vest at Brussels airport on Tuesday, left the will in the bin of the terror cell's Schaerbeek safehouse.
European security officials have been bracing for a major attack for weeks, and warned that the Islamic State group was actively preparing to strike.