Federal prosecutors announced Tuesday that the authorities had arrested two people suspected of plotting to attack "emblematic sites" in the Belgian capital following raids in the areas of Brussels and Liege.
A source close to the investigation said a counter-terrorism judge was probing links between the "Kamikaze Riders" gang and the alleged plotters.
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"The investigation is looking at this group of motorcyclists," the source told AFP on condition of anonymity without saying whether those arrested were gang member.
Belgian newspaper La Derniere Heure said that one of those arrested is a Salafist who founded the gang and that several of its members are supporters of the extremist cause.
The two suspects are due to appear in court on Thursday when a judge will determine whether to keep them in custody.
The federal prosecutor's office said one of the two was arrested on suspicion of planning attacks as well as "playing a lead role in the activities of a terrorist group and recruiting for terrorist acts."
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The second faced charges of planning and "participating in the activities of a terrorist group," it said.
It said police seized military-style training uniforms, computer hardware and Islamic State propaganda material but did not find explosives or weapons during raids around the capital Brussels and in the Liege region.
The source close to the investigation said prosecutors were trying to determine whether the alleged plotters had direct links to the Islamic State group.
Prosecutors said the police raids on Sunday and Monday was not linked to the deadly attacks in Paris in November which were claimed by the Islamic State group and which France says were prepared in Belgium.
The Belga news agency, citing an internal police memo, said there "exists a possible and credible threat of Paris-style attacks" against the high-profile Grand Place, the neighbouring central police station as well as soldiers and police in uniform.
Tourists and others flock to the Grand Place, the opulent central square of Brussels.
Belgian media reported that the city authorities will decide Wednesday whether to go ahead with a New Year's Eve fireworks display at Place de Brouckere, another central square.
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