A series of searches in the gritty Anderlecht district of Brussels netted four people and arms hidden in a garage.
A series of searches in the gritty Anderlecht district of Brussels netted four people and arms hidden in a garage, the Belgian federal prosecutor's office said.
Meanwhile, in northern France, a joint Franco-Belgian operation picked up a man on suspicion of having links to the Kamikaze Riders, a group implicated in terror offences in Belgium.
The raids and arrests come with Belgium and France still on high alert after several deadly attacks claimed by the Islamic State (IS) group, with troops on patrol in Brussels and Paris to guard key buildings and infrastructure.
Only last month, a soldier shot dead a man who had attempted to set off a bomb in Central Station, right in the heart of the Belgian capital, sparking fears that further incidents might be in the offing.
Investigators said at the time they had evidence that the suspect, a 36-year-old Moroccan national, had IS sympathies.
They also found explosive materials in a raid on his home in Molenbeek, a Brussels district where many of the jihadis who carried out the deadly Paris attacks in November 2015 and those in the Belgian capital in March 2016, grew up and found shelter.
In a statement on the latest raids, the Belgian federal prosecutors' office said "various weapons" were found during one of the house searches and that four people had been "arrested and taken in for questioning".
A source close to the probe said investigators had found at least two Kalashnikov assault rifles, while reports spoke of explosives also being discovered during operations in the immigrant-heavy Anderlecht district.
A prosecutors' spokesman said separately the raids were "directly linked" to members of the Kamikaze Riders, not to the investigation into the Paris and Brussels attacks.
The prosecutor's statement said the raids were "completely independent" from that probe.
In France, a source who asked not to be named, said a 42-year-old arrested in a suburb of the northern city of Lille was suspected of plotting a "violent action."
Several members of the Kamikaze Riders, formed in 2003 and known for testing the patience of the police, were suspected of links to foiled attacks in Brussels in late 2015.
In October 2016, two members of the group were convicted of belonging to a terrorist group, one jailed for six years, the other for three.
They were suspected of plotting an attack similar to the November 2015 carnage in Paris that left 130 people dead.
Four months later, jihadists also struck in Brussels, hitting the airport and the metro, killing 32 people and leaving hundreds injured.
The killings rocked Belgium and caused consternation when it became clear that many of those involved had been known to the authorities for some time.
Several came from Molenbeek, home to a large Moroccan immigrant community, and had hidden there after the attacks before finally being tracked down by the police.