According to the Kronen Zeitung newspaper, two gunmen attacked a Catholic church in the afternoon as gunshots were heard.
The EKO Cobra federal police anti-terrorism unit and the SWAT team of the Vienna Riot Squad had arrived at the scene of the incident, the report said.
According to preliminary data, from eight to 15 people have been injured in the attack.
Local police further said that the attackers may still hide on the premises of the church complex.
"We are talking about a large complex of buildings, at the moment it is possible that the criminals and their victims are still on the premises. The crime scene is cordoned off, our colleagues are conducting a search there," Vienna police said in its Twitter blog.