Several people were injured in an argument between two groups with firearms and stabbing weapons in Dortmund.
There was therefore a large-scale police operation on Sunday, several streets were closed.
Two groups of people clashed on a sidewalk, said a police spokesman on Sunday.
The police were able to arrest suspects shortly after the confrontation, according to a statement from the police and public prosecutor in Dortmund.
The incident occurred shortly after 3 p.m.
How many injured and arrested there were and what the background of the dispute had is, according to the information, the subject of the investigation.
"The investigation is in full swing," said the police spokesman.
In the evening the crime scene was examined.
A police spokeswoman said: "We are on site with strong forces." According to the police, there was no longer any danger for the population in the evening.
At the beginning, bystanders were asked to avoid the area around the scene of the incident.
Witnesses of the crime are asked to report to the Dortmund police station on +49 231 132-7441.
The investigators ask above all to send cell phone videos to the address nrw.hinweisportal.de.
They could be of considerable importance to the further investigation, it said.