Nizar Trabelsi, a Tunisian who once played professional soccer in Germany, was given the maximum sentence of 10 years in prison. He had admitted planning to drive a car bomb into the canteen of the Kleine Brogel air base, where 100 U.S. military personnel work.
Another Tunisian-born suspect, Tarek Maaroufi, was sentenced to six years for his involvement in the 2001 assassination of an anti-Taliban military commander in Afghanistan.
Twenty others were convicted of lesser crimes and received sentences ranging from three and one-third to five years. One defendant was acquitted.
"Terrorism has destroyed the liberty and freedom of individuals," Judge Claire de Gryse said at the end of Belgium's biggest-ever terrorism trial. "These acts must be sanctioned most severely."
Defense attorney Yves de Quyve said the court had ignored the remorse Trabelsi had shown during the trial. "They made an example of Mr. Trabelsi after the Sept. 11 attacks," he said. "We believe it was an overly severe sentence."
De Quyve told reporters he would talk with Trabelsi about a possible appeal.
Trabelsi, 33, fidgeted in his seat, smiling at times and trying to talk to his co-defendants as the judge reviewed evidence presented during the four-month trial. He was stone-faced as the sentences were read.
None of the defendants was allowed to address the court during the three-hour session.
Federal prosecutors charged the group had formed a "spider's web" of Islamic radicals plotting attacks and recruiting fighters in Europe for al Qaeda and the now-deposed Taliban in Afghanistan.
Trabelsi, who says he met Osama bin Laden in Afghanistan and asked to become a suicide bomber, was arrested two days after the Sept. 11, 2001, terrorist attacks in the United States.
His arrest led to the discovery of the raw materials for a huge bomb in the back of a Brussels restaurant.
"While bin Laden was preparing for attacks on the United States, Trabelsi with others were preparing and looking for explosives in Europe," the judge concluded.
She said phone and credit card records showed Trabelsi's links with terrorist cells in other parts of Europe. Evidence from Belgian army experts on the explosives gathered by Trabelsi showed the attack was "technically possible," de Gryse said.
Although he admitted to the Kleine Brogel plot, Trabelsi has denied allegations, made by a terrorist suspect held in France, that he also plotted to blow up the U.S. Embassy in Paris. An investigation in that case is continuing in France.
Nuclear weapons are believed to be stored at Kleine Brogel, in eastern Belgium, although officials refuse to confirm or deny their presence.
Because Belgium has no specific anti-terrorist laws, Trabelsi was charged with attempting to destroy public property, illegal arms possession and membership in a private militia.
Maaroufi, 41, was accused of involvement in a fake passport ring linked to the Sept. 9, 2001, killing of anti-Taliban leader Ahmed Shah Massood by two suicide bombers allegedly traveling on false Belgian passports. A Tunisian-born Belgian citizen he was also accused of trying to recruit for a foreign military force.
The other suspects, who are all of North African origin, faced a range of charges including forgery, conspiracy to commit a crime, handling stolen goods or membership of a private militia.
Most of the defendants claimed innocence and have said some of their suspicious contacts were maintained out of a sense of international religious brotherhood, not an attempt to commit crime or terrorism.
The trial was held under extreme security precautions at the ornate Justice Palace in the center of Brussels.
All the defendants entered the courtroom handcuffed to police officers, while people entering public galleries had to go through metal detectors. Riot police were on standby outside but there were no protests or demonstrations.
The case in Belgium is one of several terror-related prosecutions currently underway in Europe:
In London, an official said last week that police and the security services have prevented several terror attacks in London over the last 18 months, and the capital remains on a high state of alert.