Authorities said the perpetrator has sustained life-threatening injuries.
Police on Twitter told people to avoid the station.
During negotiations over the hostage, the man asked for the release of a Tunisian woman and demanded to be allowed to leave the train station unimpeded, Rueschenschmidt told reporters.
Local media reported shots being fired inside the station, something which a police spokesperson later denied. "During the police action he hurt two bystanders, one of them seriously", police said in a statement.
Heavily armed police now surround the station, which has been evacuated and closed off to the public.
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It was originally a brooch commissioned in 1830 and passed down to Queen Victoria in 1837, and then to Queen Elizabeth in 1952. Once she reached the altar and stood by his side, he removed them and handed them to best man, brother Thomas.
The suspect is probably a 55-year-old Syrian citizen, Cologne's deputy police chief Miriam Brauns said, adding that police found identification documents in the back of the pharmacy that showed that the owner of the papers was allowed to stay in Germany until mid-2021. No details have emerged yet about the hostage-taker or any potential motive.
Reports speak of at least one explosion during the incident.
German railroad operator Deutsche Bahn tweeted that some of the station's tracks were shut down temporarily and trains were being canceled or delayed.
German special Police walks near the main train station in Cologne, Germany, October 15, 2018, after the train station was closed after hostage-taking.
She was freed when police stormed the shop. "The suspect is under control", Cologne police said.