Raids took place on Sunday and Monday in Brussels and the provinces of Flemish Brabant and Liege, BBC reported.
However, the latest arrests are not linked to the November 13 Paris attacks in which 130 people were killed, prosecutors said.
One of those arrested is suspected of leading and recruiting for a terrorist cell.
The suspects are accused of planning attacks against several "symbolic targets" in Brussels, as well as on the police, according to the Belgian broadcaster RTBF.
Read More: Belgian authorities charge 4th suspect with Paris attacks
Propaganda for the Islamic State (IS) terror group was among the materials seized.
In November Brussels was placed under a four-day lockdown closing universities, schools and the metro system, amid fears of a Paris-style attack.