16 have been arrested in 19 raids across Belgium, amid fears a Paris-style mass attack may be imminent.
Salah Abdeslam was not among those arrested. The Belgian national is suspected of playing a role in the attacks that killed 130 people in Paris on November 13.
A friend said he may have a suicide vest with him.
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Armed police mounted searches on Sunday evening and cordoned off areas close to the Renaissance Grand Place around the town hall.
Helicopters could be heard flying overhead, while Police evacuated restaurants, blocked off roads and asked residents to stay away from windows.
Offices also carried out raids in Molenbeek, where Abdeslam lived, and in Charleroi, where one man was taken into custody.
The Prime minister announced another day of lockdowns in Brussels. The metro and schools are all closed.