While it is unclear how many officers are involved in the raid, a police helicopter is currently hovering over the area.
Several streets remain closed by police following the start of the raid at around 6.15pm local time (7.15pm GMT).
Belgian historian Pieter Van Ostaeyen reported the army has been deployed in the area.
He tweeted: “#Molenbeek Brussels 4 or more houses searched by police forces. Increased presence of military and police reported #Belgium”
Brussels police, however, refused to say who they were targetting in the raids.
A spokeswoman said: “Several raids are being carried out as part of an investigation into terrorist activities.”