Seven of the suspects appeared in court, while two others, who are still on the run, will be judged in absentia.
The central figure in the trial is Malika El Aroud, a 50-year-old Belgian-Moroccan.
Aroud is the widow of Abdessatar Daman, one of two men suspected of killing Ahmad Shah Massoud, a Northern Alliance commander in Afghanistan.
Massoud died in a suicide bomb explosion, two days before the September 11, 2001 attack on the United States.
According to Johan Delmulle, Belgium's federal prosecutor, Aroud led the recruitment of fighters in Belgium, sending young Muslims off to train on the Afghan-Pakistan border.
It is alleged they were sometimes escorted by her second husband, Moez Garsalloui, who is one of those being tried in absentia.
According to the prosecutor, Garsalloui had ties with "important" al-Qaeda figures.
Aroud has dismissed the prosecution case as "empty".
The prosecution evidence includes a video made by Hicham Beyayo, another of the accused.
Delmulle said Beyayo had received the "green light to carry out an operation from which he wasn't expected to return," and in the video "had said goodbye to his loved ones".
Beyayo, 24, has denied intending to carry out a "terrorist" attack.
A probe into the suspects began in late 2007, following information gleaned during investigations into an escape plan made by Nizar Trabelsi, a Tunisian.
Trabelsi was serving a 10-year sentence in Belgium for planning an al-Qaeda attack in September 2001.
Under that plan, a truck bomb was to have targetted a military base housing US troops.