In a mass brawl involving several hundred participants in the Altendorf district of Essen, a 30-year-old man was seriously injured by a stabbing weapon.
At least two other people were also believed to have been injured in the altercation on Saturday night, police said.
There are first indications that the brawl was triggered by the escalation of a dispute between extended families.
In the area between Helenenstrasse and Siemensstrasse, there were serious physical attacks between those involved, who used furniture and crockery, but also knives and sticks, among other things.
The seriously injured man was taken to hospital by ambulance.
One of the two slightly injured was already wanted by the police because of a robbery.
The man was arrested.