In Antwerp, 46 people have gone on trial suspected of being members of Sharia4Belgium, an organisation thought to be sending extremist fighters to Syria.
Most of the suspects remain at large. However Fouad Belkacem, the leader of the radical group, was among eight present in court on Monday.
He and 15 others are being tried on charges of leading a terrorist organisation.
Jejoen Bontinck, 19, is alleged to be a member of Sharia4Belgium. He claims Belkacem exerted a powerful influence over his troops and has told prosecutors he used to regard the Belgian as his “spiritual leader”.
Bontinck spent eight months in Syria between February and October 2012, before his father travelled out to bring him home.
As well as being on trial, he is also one the plaintiffs; accusing some of his co-defendants of kidnapping and torturing him in Syria.
His father, Dimitri Bontinck, spoke to press outside the court.
“No, I am not worried about my son, not at all,” he said, “because he is has three functions in this case: first, he is a suspect; second he is a victim – and the prosecutor has proof that he is a victim; and the third one, the most important, is he is a golden witness in this case. So my son is not afraid, no.”
In 2012 the group announced it was disbanding, but the authorities suspect it has continued to operate and believe dozens of militants have since been recruited.
Investigators say leader Belkacem never travelled to Syria, but claim he was the catalyst urging others to go there to fight.
If accused, he could be handed a prison sentence of up to 20 years.