Police say clashes at a Kurdish cultural festival in southwestern Germany left 80 officers injured. Thirty-one people were detained, AP reports.

Police in the city of Mannheim say officers were pelted with stones, bottles of water and other objects Saturday, Sept 8 afternoon after a security firm employed by organizers called them in to help deal with an argument over a teenager who had tried to get into the event with a banned flag. Officers used pepper spray.

The clashes lasted for about two hours. Police say 13 vehicles were damaged by stones, bricks and other projectiles.

One officer suffered rib injuries and another 79 were lightly injured.

About 40,000 people were at the festival.