At least one person was killed and several wounded in central Vienna in exchanges of gunfire late on Monday, in what the Austrian interior minister said was believed to be a terrorist attack near the central synagogue.
Vienna police said on Twitter there were multiple suspects and six different locations involved.
A large area of central Vienna was cordoned off and police said a significant deployment was under way.
Interior Minister Karl Nehammer told Austrian broadcaster ORF that the attack was believed to have been carried out by several people and that all six locations were in the immediate vicinity of the street housing the central synagogue.
"At the moment I can confirm we believe this is an apparent terror attack," he said.
"We believe there are several perpetrators. Unfortunately there are also several injured, probably also dead."
A spokesman for the ambulance service said at least one person had been killed and several injured. One of the suspects and a bystander had been shot dead and a police officer was among those injured, local news agency APA said.
"Shots fired in the Inner City district - there are persons injured - KEEP AWAY from all public places or public transport," the police said on Twitter.
Jewish community leader Oskar Deutsch said on Twitter that it was not clear whether the Vienna synagogue and adjoining offices had been the target of the attack, and said they were closed at the time.
Videos circulated on social media of a gunman running down a cobblestone street shooting and shouting. Reuters could not immediately verify the videos.
Vienna police urged people not to share videos and photos via social media. "This jeopardizes police forces as well as the civilian population," they said on Twitter.
In 1981, two people were killed and 18 injured during an attack by two Palestinians at the same synagogue. In 1985, a Palestinian extremist group attacked Vienna airport with hand grenades and attack rifles, killing three civilians.
In recent years, Austria has been spared the sort of large-scale attacks seen in Paris, Berlin and London.
In August, authorities arrested a 31-year-old Syrian refugee suspected of trying to attack a Jewish community leader in the country's second city Graz. The leader was unhurt.