Eight people were arrested in Brussels after counter-terror raids as part of an investigation into an alleged attack plot, a source close to the probe said.

All eight were arrested Sunday in Molenbeek, an immigrant district linked to the Paris and Brussels terror attacks, following the raids, the source told AFP.

The eight were taken for questioning before a judge investigating an alleged terror plot, the source said, confirming a report in the Belgian daily La Derniere Heure.

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The source said investigators suspected an attack was in preparation, but gave no other details.

The judge will decide late Monday whether to formally charge them and whether to continue holding the suspects or release them, the source added.

The arrests took place following raids in Molenbeek, but also in Geraardsbergen and Mechelen, both in northern Dutch-speaking areas of Belgium.

Molenbeek was home to several members of the cell that carried out the November 13, 2015 attacks on Paris and the March 2016 suicide bombings in Brussels.