The operation, which was launched against a suspected extreme-right group, raided a dozen addresses around the northern city of Antwerp.
One person has died in a shootout with Belgian police during raids on a suspected extreme-right group, prosecutors have said.
The operation, targeting a dozen addresses around the northern city of Antwerp, was launched to counter "the preparation of a terrorist attack".
In a statement prosecutors said "many weapons and ammunition" were seized in the raids. Some of the weapons were legally registered, they said on Wednesday.
During one raid, "an exchange of gunfire occurred between law enforcement and one of several people inside a building. This person died."
The statement gave no further details about the shootout or the targeted group.
Last weekend, four Dutch suspects were arrested over what prosecutors described as a plan to kidnap Belgium's justice minister, Vincent Van Quickenborne.
It was not known if that incident was linked to Wednesday's raids on addresses in Antwerp and its suburbs, the nearby village of Kasterlee and the city of Ghent.