The organizers called for people to wear white ribbons to declare their opposition to the denial of genocide after the families of Bosnian Muslims and Croats killed in the area were denied a public memorial last week by the town's current mayor, Marko Pavic.
Pavic said such an event would "undermine the town's reputation." He rejected the use of the term "genocide" by victims groups and said they had politicized the commemoration.
During the war, Prijedor's ethnic Serbian authorities forced the non-Serbian residents to put white flags on their houses and wear white ribbons when leaving home.
Over 3,000 Bosnian Muslims and Croats died in the area in northwestern Bosnia during the war, including about 250 women and 100 children.
The former head of the Prijedor district, Milomir Stakic, was sentenced to 40 years in prison in The Hague in 2003 for crimes in the area.