The Cologne main rail station was evacuated on Monday afternoon after a woman was taken as a hostage by a man in a pharmacy, the police said on Twitter.
The local Cologne City Information quoted the police as saying that a person is armed in the pharmacy.
The Focus Online news quoted a police spokesman as saying that the police have been contacting with the hostage-taker.
The police said on the Twitter that they have deployed strong forces at the scene and urged people to avoid the area.
Witnesses reported that gunshots were fired, while the police could not confirm this so far.
Authorities have ordered an evacuation of the station, which is a major hub for rail traffic across Germany and western Europe. The police also called on the public not to publish or share images or video of their positions.
The German railway operator Deutsche Bahn announced on Twitter that it closed the station and canceled some trains as a result of the police investigation.