The public have been told to avoid the centre of the city.
The official Twitter account for the Viennese Police said: "Shots fired in the Inner City district - there are persons injured - KEEP AWAY from all public places or public Transport - don't share any Videos or Fotos [sic]!"
The suspect is reportedly on the run with Viennese police launching a major manhunt according to some local outlets.
Others have reported that the alleged gunman has blown himself up after detonating a suicide vest, but that other perpetrators remain at large.