Seven people were "taken in for further investigation" during six raids linked to French national stadium bomber Bilal Hadfi, the federal prosecutors office said in a statement.
The other two arrests were also linked to last Fridays attacks, the federal prosecutors office said in a statement.
Many of the raids were in the largely immigrant area of Molenbeek, where Paris attacks ringleader Abdelhamid Abaaoud and two brothers linked to the attacks, Brahim and Salah Abdeslam, came from.
But there were also raids in the upscale Brussels suburbs of Uccle and Laeken.
Prosecutors said they would give more details on how long the suspects would be held on Friday.
Belgian police have carried out a series of raids since the Paris massacre in which 129 people died, with Belgium facing increasing criticism for having failed to stop the attackers.
A major operation targeting Salah Abedeslam in Molenbeek on Monday failed to find the suspect.
Two people were charged with involvement in terrorism on Monday in relation to the Paris attacks.(*)
Editor: Heru Purwanto
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