Police raided seven homes on Sunday in the Brussels district of Molenbeek as well as the Flemish towns of Geraardsbergen and Mechelen, the office of the federal prosecutor said, adding that no weapons or explosives had been found.
"A judge specialised in terrorism matters had asked for the home searches to be carried out," the office said.
Newspaper DH had earlier reported that those being held were suspected of planning a militant attack.
Prosecutors did not give any details, but said the case was not connected to the attacks in Paris of November 2015 - which involved Belgian as well as French gunmen - or the Brussels bombings of March 22.
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