Bild daily and the Frankfurter Allgemeine Zeitung reported that elite police were on their way to the scene.
The Darmstaedter Echo said 20 to 50 people had been injured. CNN reports that the attacker has been killed and efforts are being made to unravelled his identity as well as establish his motivation.
Police were not immediately available to comment. Details are still sketchy as security operatives are on ground to monitor the situation.