One of the policewomen was taken to hospital with “deep wounds to the face” while the other was slightly injured, Belga news agency said. The incident took place outside the main police station.
Belgium has been on high security alert for months since suicide bombers hit Brussels airport and a subway station near the European Union’s institutions on March 22, killing 32 people.
The attacks were claimed by the Islamic State terrorist group.
Separately on July 30, Belgian authorities charged a 33-year-old man, identified as Nourredine H., with attempting to commit “terrorist murder” and “taking part in the activities of a terrorist organisation.”
Belgium was also a springboard for the Islamic State cell which carried out the November 13 2015 attacks in Paris which killed 130 people, investigators say.