Two people including one attacker were killed in central Vienna on Monday evening in what Austrian Chancellor Sebastian Kurz described as a "repulsive terror attack".
Here are the main reactions so far to the shootings, which police said began at around 8:00 pm (1900 GMT) and took place at six different locations:
"We are experiencing difficult hours in our republic," Chancellor Kurz said on Twitter.
"Our police will act decisively against the perpetrators of this repulsive terror attack. We will never be intimidated by terrorism and we will fight this attack with all means."
French President Emmanuel Macron said: "We French share the shock and sorrow of the Austrian people following the attack in Vienna."
"After France, it is a friendly nation that has been attacked. This is our Europe. Our enemies must know who they're dealing with. We will concede nothing," he tweeted in both French and German.
Germany vowed not to "give in to hate that is supposed to divide our societies".
"Even if we can't yet foresee the extent of the terror, our thoughts are with the wounded and the victims in these difficult hours," the foreign ministry wrote on Twitter, calling the news from neighbouring Austria "horrifying and disturbing".
British Prime Minister Boris Johnson tweeted that he was "deeply shocked" by the night's events and that the "UK's thoughts are with the people of Austria - we stand united with you against terror".
Italian Prime Minister Giuseppe Conte said there was "no room for hatred and violence in our common European home", while his foreign minister Luigi Di Maio tweeted that "Europe must react".
European Council chief Charles Michel said: "Europe strongly condemns this cowardly act that violates life and our human values. My thoughts are with the victims and the people of #Vienna in the wake of tonight's horrific attack. We stand with Austria."
EU Commission President Ursula von der Leyen tweeted that she was "shocked and saddened", and that her "thoughts are with the families of the victims and the Austrian people".
The president of the European Parliament, David Sassoli, said he felt "sadness and horror" and the EU's top diplomat Josep Borrell called it "a cowardly act of violence and hate".
In the Czech Republic, which neighbours Austria, Prime Minister Andrej Babis said he was "horrified by the attack" and wanted to "express my solidarity to all people in Austria and my friend Sebastian Kurz".
Czech police said they had started random checks on the country's border with Austria.
"Police are carrying out random checks of vehicles and passengers on border crossings with Austria as a preventive measure in relation to the terror attack in Vienna," Czech police tweeted.
Police also said they had stepped up "supervision over major Jewish facilities in the Czech Republic" in a preventive measure that "reflects developments not only in neighbouring Austria".
Greek Prime Minister Kyriakos Mitsotakis tweeted that he was "shocked by the horrific attacks in #Vienna" and had offered Kurz Athens' "full solidarity".
"Our thoughts are with the people in Vienna and the authorities dealing with the situation. Our hearts, with the victims and their loved ones," Mitsotakis added.
Australian Prime Minister Scott Morrison tweeted his shock over the "awful terror attacks in Austria".
He said: "The situation remains fluid and details of the attack are still not clear," and offered the Austrian Chancellor and the country's people his thoughts and assurances.