The two men, named as Nourredine H., 33, and his brother Hamza were detained after house searches in the Mons region and the city of Liege, BBC reported.
No weapons or explosives were found in the searches.
"Based on provisional results from the investigation, it appears that there were plans to carry out attacks in Belgium," BBC quoted the federal prosecutor's office as saying in a statement.
But the statement added that as yet no link had been found to the Brussels airport and metro station bombings in March that killed 32 people.
A judge would decide on Saturday whether to keep the suspects in detention, the statement added.
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