Belgian police have detained 14 people suspected of being members of the al-Qaeda network.
Officials said they include a man believed to have been about to launch a suicide attack.
Federal prosecutor Johan Delmulle said police did not know where the attack was to have targeted.
The detentions came as a two-day summit of European Union leaders was due to start in the Belgian capital, Brussels, on Thursday afternoon.
A total of 242 police officers carried out 16 raids in Brussels and one in the eastern city of Liege.
Police seized computers, data storage equipment and a pistol during the raids, reports say, and the men and women arrested are due to appear before anti-terrorism judges later.
Mr Delmulle said the suspects could have been targeting Pakistan or Afghanistan, "but it can't be ruled out that Belgium or Europe could have been the target".
He said the man suspected of planning the suicide attack had "received the green light to carry out an operation from which he was not expected to come back."
"He had said goodbye to his loved ones, because he wanted to enter paradise with a clear conscience," he added.
"This information, linked to the fact that the EU summit is being held in Belgium at the moment, left us with no choice but to intervene today."