An investigation allegedly revealed “the threat of serious attacks that would target several emblematic places in Brussels and be committed during the end-of-year holidays,” a spokesman said.
A source said the Belgian capital’s main square, packed this time of year with shoppers and tourists, was one of the suspected targets.
“On the Grand Place, there are a lot of people, as well as patrolling soldiers and police [and] a police station nearby,” said the source.
The two male suspects were arrested following searches on Sunday and Monday in the Brussels area, the Liege region and Flemish Brabant, the prosecutor’s office said.
One was charged with leading a terrorist group planning to commit terrorist offences, the other with participating in a terrorist activities as a principal actor or co-actor.
During the searches, military-type uniforms, propaganda materials from the Islamic State group and computer material were seized. However, no weapons or explosives were found.
Six people were taken in for questioning, but four were released.
The prosecutor’s office said the probe was not connected to the 13 November attacks in Paris, despite numerous suspects, including presumed ringleader Abdelhamid Abaaoud and fugitive Salah Abdeslam, having connections to Belgium.
On 21 November, after the Paris attacks that killed 130 and injured hundreds, the terrorist alert level for all of Brussels was temporarily raised to its maximum level.
Police and soldiers have been ordered to take special precautions to ensure their own safety, said Benoit Ramacker, for the government Crisis Centre.
Police and army patrols were reinforced in Brussels following the Paris attacks, and Mr Ramacker said a new threat assessment on Monday after the latest arrests concluded officers and soldiers might become targets “in the exercise of their functions”.
In January, Belgian anti-terrorism units broke up what they said was an imminent attack on police by raiding a house in the eastern city of Verviers, killing two suspected jihadis and arresting a third.
Their main quarry, however, was not there: Abdelhamid Abaaoud, who went on to become the suspected ringleader of the 13 November attacks in Paris.