The defendants face charges in connection with plans to attack the Brussels metro.
One of the accused is the widow of the Afghan opposition leader, Ahmed Shah Massood.
Massood was killed just days before the 11 September attacks in the United States.
Massood's widow is still in detention as is one of the other defendants.
Seven others have been released.
Ring leaders risk twenty years in gaol. The others face sentences of up to ten years.