Authorities on Wednesday called off the usual New Year´s Eve fireworks display in the capital, citing fears of a possible militant attack.
Police had detained six people after searches at six locations in the Belgian capital and one just outside the city.
They also seized computers, mobile phones and equipment for airsoft, a sport using guns that shoot non-lethal plastic pellets.
Three of the six were released later on Thursday.
Prosecutors said they were holding the other three for a further 24 hours.
In the same investigation, two Belgian nationals arrested earlier this week and named as 30-year-old Said S. and 27-year-old Mohammed K. , are being held on suspicion of planning a terrorist attack, prosecutors said.
A court on Thursday extended their custody for a month.
Both are members of the Kamikaze Riders, a Brussels-based group that carries out motor bike stunts which can be seen in online videos.
Most of the members are of North African origin.
Some Belgian media say the group harbours Islamic State sympathisers and that the search for other group members had led Brussels to cancel Thursday night´s planned firework display.
Belgium has been on alert level three – which means a possible and probable serious threat – since the Nov. 13 Paris attacks.
The capital itself was on the maximum alert level four for almost a week last month.
The presumed ringleader of the Paris attackers, Abdelhamid Abaaoud, was Belgian and grew up in the poor Brussels district of Molenbeek.
Two more of the Paris attackers had been living in the Belgian capital.
Federal prosecutors said on Thursday that a 10th person had been charged with involvement in the attacks after around 10 mobile phones were found during a search at a Molenbeek residence on Wednesday.