The Belgian authorities have charged six people for suspected membership of al-Qaeda, including a woman whose husband was involved in the assassination of an Afghan commander opposed to the Taliban.
Lieve Pellens, a spokeswoman from the federal prosecutor's office, said the six constituted the hard core of a "terrorist" group and included one person who was allegedly plotting a suicide attack.
The suspects had been arrested in police raids on Thursday, hours before the start of a European Union summit of 27 government leaders in the Belgian capital.
Eight other suspects had been picked up, but a judge decided that there was insufficient evidence to hold them.
All suspects under arrest are Belgian, and include Moroccan-born Malika El Aroud, whose first husband died in a 2001 suicide attack in Afghanistan that killed Ahmed Shah Massoud, who had been fighting against the Taliban.
Most of the other suspects are in their 20s or early 30s and only one of those was known from other police investigations, Pellens said.
El Aroud was detained in a raid in December last year, but was released because of insufficient evidence.
Pellens said that despite a year-long investigation, it remained unclear whether any attack was imminent.
Nearly 250 police officers raided 16 locations in Brussels and one in the eastern city of Liege early on Thursday, confiscating computers, data storage equipment and a pistol.
Police said they considered that they had to move at that point because it was too risky to have the suspects at large when the EU summit opened.
Pellens said it was unlikely, though, that the suspects would have picked such a high-security target.
She said a possible attack might have been planned in Iraq, Pakistan or a European location.
Johan Delmulle, a federal prosecutor, said one of the suspects had recently "said goodbye to his loved ones because he could go to paradise with a clear conscience".
Investigators waited a year before moving in, opting to detain the entire alleged cell rather than a single part.
The investigation centred on people linked to Nizar Trabelsi, a 37-year-old Tunisian sentenced to 10 years in prison in 2003 for planning to drive a car bomb into the cafeteria of a Belgian air base where about 100 American military personnel were stationed.
Security services in several European nations suspect Trabelsi, who trained with al-Qaeda in Afghanistan, had links with groups in Britain, France and elsewhere in Europe.