He said parts of the station, one of the biggest in the country, were closed off as well as Breslauer Platz square behind the station.
German police say they now have an armed assailant at Cologne's main railway station "under control" and the woman he took hostage is receiving medical treatment.
There had been reports of shots fired, although police later said this was not the case.
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Police refused to speculate whether the incident was terror related, said DW's David Martin.
Police tweeted that the hostage was slightly injured and was now being treated.
State rail operator Deutsche Bahn suspended all train traffic through Cologne, a major hub for rail travel through North Rhine-Westphalia state and nationwide.
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Gilles said specially-trained officers were talking to the hostage taker, who hadn't been identified.
Two explosions were heard in the area during the hostage rescue not long after elite counter-terrorism team (SEK) rushed to the pharmacy building, NTV broadcaster reported.
Cologne police urged people to stay clear of the train station and asked that no photos or videos of the hostage situation to be posted on social media. The hostage sustained light injuries and was taken care of.
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