Belgium has been on high alert since the Paris attacks that left 130 people dead and the Brussels attacks that killed 32 people.
The raids are not related to either the Islamic State attacks in Paris in November 2015 or the bombings in Brussels in March last year, a spokesman for the federal prosecutor's office told AFP.
"There were eight house searches in the Brussels region," the spokesman said, adding the operation was ongoing and there would be a statement around noon (1100 GMT).
Belgium has been on high alert since the Paris attacks that left 130 people dead and the Brussels attacks that killed 32 people. Investigators say the same Brussels-based cell carried out both attacks that were claimed by the Islamic State group.
The website of the newspaper La Derniere Heure first reported the raids, saying special forces backed up counter-terror police who raided homes in Molenbeek, Schaerbeek, Anderlecht and Laeken.
It said several people were detained for questioning on suspicion that they had either travelled to Syria or returned from the war zone.
Belgium is the EU country with the highest per capita number of fighters who have gone to wage jihad in Syria and Iraq, a figure estimated at 465.