"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".