Malika El-Aroud is the chief defendant. She is a 50-year-old woman, who stands accused of recruiting youngsters for a Jihad or Holy War.
El-Aroud allegedly recruited youngsters in Brussels together with her husband, who is still on the run.
The youngsters were packed off to training camps in Afghanistan where they were trained to carry out suicide bomb attacks.
El-Aroud is the widow of the killer of the Afghan opposition leader Ahmed Shah Massood. Massood was assassinated days before the 9/11 attacks on America in 2001.
The trial initially started in January, but at this time only the timetable for the proceedings was set out.
Monday's start of the trial proper descended into farce as the defendants incurred difficulties reaching the court room due to a prison guards' strike.
At the courtroom stringent security measures were in place. All those present had to pass through a metal detector and were frisked.
The trial is expected to last for several weeks. Key defendants risk ten years in prison, but maintain their innocence and claim they did not have any definite plans for terrorist outrages.