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According to German media, over 20 people were injured. Police have not confirmed any casualties yet.
The man, who was wearing a mask, carrying a gun, and wearing a cartridge belt around his shoulder, reportedly fired a shot in the air before barricading himself in the building.
Dozens of people fled the cinema. It is not yet known if there are any casualties or if the man has taken hostages.
In response, German police triggered a major alarm, sending a special commando team and helicopters to the scene, and police have cordoned off a wide area around the cinema.
Germany cinema shooting: Between 20 and 50 people wounded in Viernheim cinema shooting pic.twitter.com/fY6yDfs3AW
This story is breaking and will be updated as more information is available.