A 32-year-old man was seriously injured by a group of men with a hammer and a knife in a schoolyard in Hagen.
He was taken to a hospital, police reported on Monday.
The passer-by was therefore stopped and pushed by five to six men on Sunday "for no apparent reason" as he walked across the schoolyard.
One hit him on the head with a hammer, then another stabbed the 32-year-old's hand with a large knife.
Hagen - The group then fled.
The police asked for evidence on Monday.
dpa