Fourteen suspected al-Qaeda members, including a man believed to have been planning a suicide attack, have been arrested by Belgian police as European Union leaders gathered in Brussels for a key economic summit.
"We don't know where the suicide attack was to take place," Johan Delmulle, a federal prosecutor, said on Thursday.
"It could have been an operation in Pakistan or Afghanistan, but it can't be ruled out that Belgium or Europe could have been the target."
Delmulle said one suspect had "received the green light to carry out an operation from which he was not expected to come back" and "had said goodbye to his loved ones, because he wanted to enter paradise with a clear conscience".
"This information, linked to the fact that a European summit is getting under way at this moment in Brussels, left us no choice but to take action today," the prosecutor said.
Delmulle said the raids were linked to a similar pre-Christmas sweep last year.
About 250 police officers carried out raids 16 raids across Brussels and one in the eastern city of Liege on Thursday, confiscating computers, data storage equipment and a gun.
The federal prosecutor's office described the raids as "the most important" anti-terrorism operation in Belgium, and said it was linked to a Belgian Islamist group involved in training as well as fighting on the Pakistan-Afghan border, in liaison with "important figures" in al-Qaeda.
In a statement, the country's justice and interior ministers said: "It is now clear to all that we were dealing with a real risk. It is more than likely that an attack in Brussels has been prevented".
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