A total of eight people are now being held in the case, including two men who have been formally charged with terrorism-related offences following their arrests earlier this week, the federal prosecutor's office said.
The six new suspects were detained following seven police raids in and around the Belgian capital in which computer equipment, mobile phones and "airsoft material" were seized, it added. Airsoft is a type of airgun.
An investigating judge will decide later Thursday whether to keep the six in custody or release them, the prosecutor's office added.
The city of 1.2 million people has been on high alert since France said several men involved in the deadly November 13 attacks in Paris had links to Brussels, which is home to the headquarters of the European Union and NATO.
The prosecutor's office said the two men already formally charged over the end-of-year plot had been ordered by a court to be detained for another month.
Mohammed K, born in 1988, faces similar charges.
Their lawyers told reporters outside the courthouse that their clients denied the charges.
Belgian paratroopers wearing red and green berets and clutching automatic rifles joined police in guarding the courthouse where they appeared before a judge.
