The trial opened on Thursday of 32 people accused of running or taking part in one of Belgium's largest networks to send jihadists to Syria, AFP reported.
Belgian media said, however, that about half the defendants are still in Syria or dead.
Belgian police smashed the network after opening probes in 2012 and 2013 that led to 55 raids and the detention of 74 people.
One of the leaders of the network is Abdelhamid Abaaoud, a 27-year-old Belgian of Moroccan origin who remains at large after slipping through a Belgian police dragnet in a separate case.
Abaaoud is considered by investigators to be the brains behind a cell that police smashed in January, thwarting an alleged plot to mount attacks against the police.
Police shot and killed two of the suspected militants during the raid in Verviers, eastern Belgium.
In February, Abaaoud, who was reported at one time to be in Greece, claimed responsibility for the plot in Belgium and said he had joined the Islamic State (ISIS) group in Syria.
Abaaoud was also accused of kidnapping after his younger brother Younes travelled to Syria in January 2014 at the age of 13 and earned the media nickname of "the youngest jihadist in the world."
The second key defendant is Khalid Zerkani, a 41-year-old resident of Brussels who allegedly sent people to Syria to wage armed jihad through sermons at makeshift mosques.
At the hearing, Zerkani denied recruiting young Belgians for jihad, the Belga news agency said.
Other defendants are considered either recruiters or fighters. A number of women are among the accused for allegedly marrying jihadists in Syria or sending them money.
The verdict is not expected before the end of May.
In February, a court in the northern city of Antwerp sentenced the leader of an Islamist group that sent jihadists to Syria to a 12-year jail term and imprisoned several other members in one of the largest cases of its kind in Europe.
AFP contributed to this report.