"There was no other choice than to intervene today," federal prosecutor Johan Delmulle told reporters. He said one suspect had recorded what looked like a martyrdom video, including a farewell message.
"It is clear that we have to take the terror threat seriously," Prime Minister Yves Leterme said as he entered the EU summit building.
Helicopters flew overhead and police guarded dozens of motorcades traveling to the summit cordon. The terror-alert level in Belgium was not raised, however, officials said.
Delmulle said it was unclear where the attack had been planned to take place. Some suspects had traveled to both Pakistan and Afghanistan, and it was possible the suicide bombing might have been drawn up there. One who had recently returned "said goodbye to his loved ones because he could go to paradise with a clear conscience," Delmulle said.
Thursday's raids were linked to a similar pre-Christmas sweep last year and Delmulle said the investigation showed at the time "a group of people were in Brussels with the task of committing an attack."
Investigators waited a year before moving in - opting to ferret out the entire cell rather a single part.
"It is now clear to all that we were dealing with a real risk," the justice and interior ministers said in a statement.
"It is more than likely that an attack in Brussels has been prevented" during that time.
Claude Moniquet, the president of the Brussels-based European Strategic Intelligence and Security Center think tank, said he was convinced that "the target was located somewhere in Europe, maybe even in Belgium and not in Afghanistan."
"Some of the terrorists who were arrested were just back from that country after they had received training; they would not go there anymore," he said.
The investigation centered on people linked to Nizar Trabelsi, a 37-year-old Tunisian sentenced to 10 years in prison in 2003 for planning to a drive a car bomb into the cafeteria of a Belgian air base where about 100 American military personnel are stationed.
Security services in several European nations suspect Trabelsi, who trained with al-Qaida in Afghanistan, had links with extremists in Britain, France and elsewhere in Europe.
At the time of last year's arrests, authorities tightened security, warning of a heightened threat of attacks despite the arrests. Police stepped up patrols at Brussels airport, subway stations and the downtown Christmas market, which traditionally draws large crowds of holiday shoppers.
Leterme told reporters that the investigation justified the extreme security measures that were taken over the past year.
Authorities did not give a rundown of all the people under detention.
But Moniquet and national media said they included Moroccan-born Malika El Aroud, a 48-year-old Belgian who writes online in French under the name of Oum Obeyda.
"She is extremely active as a Jihadist who motivates" terrorists, Moniquet said. "She was writing online as recently as three weeks ago. She is very dangerous."
He did not elaborate on how he knew she had been detained.
El Aroud, who moved to Belgium from Morocco when she was very young, began writing online after her first husband died in the suicide attack in Afghanistan that killed anti-Taliban warlord Ahmed Shah Massoud.
