German police have deployed a Special Forces unit to central Cologne following a shooting incident that took place near the central railway station.
At least one person found in possession of a firearm was detained.
Up to three other suspects might be on the run, according to German media.
The incident followed an altercation involving several people, who fled on foot and in a car, the police said. The detained person was trying to escape on foot.
Police have raided a nearby residential building where some of the people involved might have been hiding. They did not find any evidence that anyone else was involved, police said on Twitter, adding that most security cordons in the area were lifted.
There have been no reports on any people killed or injured.
Some reports, citing police, suggest the incident might be linked to a row between biker gangs.