Police stopped the 14-year-old at the entrance to cultural festival on the city's Maimarkt because he was carrying a forbidden flag, the Süddeutsche Zeitung newspaper reported on Sunday.
After security guards unsuccessfully tried to block him from entering the festival's grounds, they called the police for backup.
Before long around 2,500 Kurds were in an hours-long stand-off with 600 police officers, the paper reported. The police spokesperson said the “outbreak of violence was enormous,” and added that he had never experienced something like it in his 30 years on the force.
Hundreds, “if not more than a thousand” people ran at the police, some throwing stones, water bottles, bricks and fireworks, a police spokesman told the paper.
The police used pepper spray and confiscated flags and T-shirts of banned organizations, along with four knives and a set of brass knuckle dusters.
The police spokesman said those being violent and rushing the police were supported by thousands of their fellow participants, and that they had “no chance” of calming the crowd. The area eventually cleared at around 8 pm.
The news outlet Tagesschau.de reported early indications that the violence could have been influenced by targeted propaganda, such as a rumours among the Kurdish participants that on Friday night police in Mannheim had mistreated a Kurdish demonstrator.
Various smaller incidents were reported during Friday as the Kurds gathered in Mannheim. Police stopped a march on Friday by a Kurdish youth group after some involved attacked Turkish-looking passersby.
One group waved a banned PKK flag and shouted slogans in support of the group banned and regarded by the authorities as terrorists.
Reinhold Gall, interior minister of the state of Baden-Württemberg said he was shocked by the incident, and that such events would have to be checked more closely before being allowed in the future, if they are allowed at all, Tagesschau reported.