Police detained six people for questioning over an alleged plot to strike "emblematic sites" in Brussels during the year-end festivities.
Belgian police were holding five people early Friday over an alleged New Year plot in Brussels and also arrested a 10th suspect over last month's attacks in Paris.
Both Brussels and Paris cancelled annual New Year's Eve fireworks displays as soldiers and police ramped up security in European capitals over perceived terror threats.
There were instead plenty of private firework displays throughout the Belgian capital at midnight, accompanied by cheers and loud music, but the celebrations were vastly diminished compared to last year's.
"People are scared. They are staying home and avoiding public places," taxi driver Abdullah told AFP in the Ixelles neighbourhood as he waited for customers who he estimated have dropped 40 percent this year.
Police detained six people Thursday for questioning over an alleged plot to strike "emblematic sites" in Brussels during the year-end festivities.
But three of them were released after interviews and the others were ordered provisionally held for another 24 hours as the investigation continues, the federal prosecutor's office said.
The prosecutor's office also said two men, whom they identified as Said S., 30, and Mohammed K., 27, had already been formally charged with terrorism-related offences and remained in custody.
The men's lawyers said their full names are Said Saouti and Mohammed Karay, whose arrests were announced Tuesday following police raids in and around Brussels and the eastern city of Liege.
The men denied the charges against them, the lawyers added.
Police seized military-style training uniforms, computer hardware and Islamic State propaganda material in the earlier raids. In Thursday's raids, they found computer equipment, mobile phones and airgun equipment, the prosecutor's office said.
A source close to the investigation told AFP that officials were trying to determine whether members of a motorcycle gang called the "Kamikaze Riders" were involved in the year-end plot.'Terrorist murder'
Brussels has been on high alert since it emerged that several of the attackers involved in the Paris carnage on November 13 had links to the Belgian capital.
Prosecutors said Thursday they had arrested a 10th suspect over the attacks in Paris claimed by the Islamic State group that killed 130 people and wounded many more. They charged him with terror offences.
The Belgian national, identified only as Ayoub B., was detained on Wednesday during a raid on a house in the troubled Brussels immigrant neighbourhood of Molenbeek, the federal prosecutor's office said.
He has been charged with "terrorist murder" and involvement in the activities of a terrorist group, it said, adding that he would appear in court again within five days for a custody hearing.
Molenbeek is home to the Paris attacks fugitive Salah Abdeslam and has served in the past decades as a haven for jihadists.
Brussels Mayor Yvan Mayeur said Wednesday that fireworks and related events in the central square of Place de Brouckere had been cancelled because it was not possible to "guarantee that we can check everyone coming to the event".
Last year, some 100,000 people turned out to watch the traditional New Year's Eve fireworks display.'Terrorists are winning'
Alex, another taxi driver, told AFP that "the terrorists are winning" as business had dropped by about half this year while some cafe owners said they were closing several hours early for lack of customers.
It is not the first time the official New Year's Eve fireworks have been cancelled as the authorities made the same decision in 2007 because of another terror threat.
Officials said the plot revealed Tuesday was not directly linked to the wave of bombings and shootings on a Paris concert venue, bars, restaurants and the national stadium.