Police found military clothing and ISIL propaganda material but no weapons or explosives, prosecutors said on Tuesday.
"Our investigation revealed serious threats of an attack on symbolic places in Brussels during the celebrations for New Year's Eve," the prosecutors said in a statement, according to Reuters.
A total of six people had been taken in for questioning during house searches in Brussels, a neighboring province and Liege, but four of them had since been released, the prosecutors added.
Belgium has been at the heart of investigations into the Paris attacks on November 13 that left 130 people dead after links with Brussels emerged. Two of the Paris suicide bombers, Brahim Abdeslam and Bilal Hadfi, had been living in Belgium.
Belgian police have arrested nine people in connection with the Paris attacks. But the prosecutors said Tuesday's announcement was not linked to that investigation.