The European Commission, the high-profile building that houses several thousand officials of the 28-nation European Union, was among the targets of ISIS linked militants returning from Syria, according to media reports.
The Belgian authorities have arrested several people during a crackdown. Two arrested people were from The Hague, Dutch public broadcaster NOS reported.
"They were planning an attack. One of the targets was the European Commission building. Individual commissioners were not particularly targeted. It would be more like the Jewish museum attack with the aim to kill as many people as possible," NOS reported, citing an informed source.
Belgian police reportedly found guns and bulletproof vests during the crackdown. The arrests and seizure of arms resulted from simultaneous home raids in Brussels and The Hague.
One raid was reportedly carried out in The Hague's Schilderswijk district, where a demonstration in support of the Islamic State recently took place.
ISIS is an al-Qaeda splinter group and it has seized hundreds of square miles in Iraq and Syria.
Al-Qaeda has distanced itself from the group, chiding it for its lack of teamwork in its aggressive, brutal expansion.
ISIS has in its ranks hundreds of European fighters.
Belgian daily L'Echo said the arrests come as authorities are investigating extremism in the region.
According to reports, about 400 Belgians were fighting in Syria alongside jihadists and some of them have returned home.
"Our starting point is that among them, one out of nine aim(s) to carry out an attack," L'Echo's reported, citing an unnamed source.
"We are working full time on the problem of the returning militants," Belga news agency cited a federal judicial spokesman as saying.
"We work together with the security services and that led us to carry out several operations," the spokesman said.
