A police officer has been shot and is one of those who is seriously injured, the APA news agency has said, citing Austria's interior ministry.
Viennese police on Twitter warned locals to stay away from public places and public transport.
Karl Nehammer described the incident as a terror attack and said that one attacker had died in the Austrian capital.
The Austrian Interior Ministry said the incident may be classified as a terrorist attack.
"There are several injured persons", police said tweeted. They also called on the public to refrain from posting videos or photos on the internet, saying they posed a danger both to police and civilians.
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The OE24 broadcaster reported that about 50 shots were fired in the street near the synagogue, adding that the shooting was carried out with the use of a machine gun.
The area where the shooting took place is a busy district packed with bars and restaurants, near Vienna's main synagogue, Seitenstettengasse Temple.
Austrian police said that "Currently there is a major police operation in the Inneren Stadt". We are on duty with all possible forces.
The synagogue was already closed at the time of the shooting around, Deutsch tweeted.Rabbi Schlomo Hofmeister told The Associated Press that he saw at least one person fire shots at people sitting outside bars in the street below his window.
Breaking news in the past hour reports a terrorist attack now underway in Vienna.
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