BRUSSELS, Jan. 22 (Xinhua) -- Six people were injured in a knife attack on Thursday evening in the Belgian city of Antwerp, two of them in critical condition, local media reported.

The incident occurred near the city's Opera House, where a Kurdish demonstration involving about 50 people was taking place. The gathering was initially peaceful, but a knife attack took place at around 7:20 p.m. (1820 GMT), reported the Belga News Agency.

According to Antwerp police spokesman Wouter Bruyns, four suspects have been arrested and all the injured have been taken to the hospital for treatment.

The motive for the attack remains unclear, and a full investigation is underway. ■