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.
Six people were being held on Thursday for questioning over an alleged New Year attack plot in Brussels that led to the cancellation of a fireworks display and other festivities.
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.
On Wednesday, Belgian authorities cancelled the annual New Year's Eve fireworks display and other events in the heart of Brussels after revealing an alleged jihadist plot to attack "emblematic sites" in the capital during the holiday.
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. It said Said S, born in 1985, has been charged with "threatening terrorist attacks" as well as playing a leading role in the activities of a terrorist group and recruiting potential attackers.
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.
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