The Federal Prosecutor’s Office said six houses in the greater Brussels area were searched in the operation.
No arms or explosives were found and the prosecutor’s office said no further information would be made public about the six people detained or the items seized in the police searches.
In August 2015, a man on a Thalys train that had just crossed into France from Belgium tried to open fire with a rifle but was overpowered by three Americans, two of them off-duty members of the US armed forces.
French police termed it an Islamic extremist attack, but the gunman, Ayoub El Khazzani, maintained he wanted to commit robbery.
French authorities have linked El Khazzani to Abdelhamid Abaaoud, the ringleader of the IS cell that attacked Paris and Brussels.
Brussels was on high alert yesterday with increased security after another police sweep over the weekend that left three in jail facing terrorism charges, including relatives of two of the suicide attackers who struck Brussels targets on March 22.
The MIVB metro system said a half-dozen subway stations had reduced entry on request from authorities, but all subway lines in the Belgian capital were running in the morning rush hour.