Belgian police have conducted 14 house searches in a terrorism investigation, the federal public prosecutor's office said on Thursday, adding seven people were taken in for questioning.
"They are suspected, among other things, of preparing a terrorist attack. The specific targets of the attack had not yet been determined," it said in a statement.
A judge will decide later if they are to be charged.
The house searches took place in the cities of Antwerp, Liege and Ghent, among others, and in the Brussels region.
The prosecutor's office did not immediately comment on whether the plans had a link to the Paris Olympics, which commences on Friday.
The Paris anti-terrorism prosecutor did not respond when asked if it was involved in the investigation.
Links to prior attacks
The perpetrators of the 2015 Paris attacks, in which 130 people were killed and 368 wounded, largely planned and coordinated them from Belgium, with several of the attackers being Belgian nationals or residents.
In 2016, bombings at Brussels airport killed 34 people and injured 340. Among those convicted for the attacks was Salah Abdeslam, who was also the main suspect in the Paris attacks trial.