Condolences poured in from around the world, with top officials from the European Union, France, Norway, Greece and the United States expressing their shock at the terror attacks in Vienna.
President Emmanuel Macron of France, which has seen two deadly knife attacks in Paris and Nice in recent weeks, issued a statement expressing shock and sorrow.
"This is our Europe," he said. "Our enemies must know with whom they are dealing. We will not retreat."
US President Donald Trump said in a tweet that "our prayers are with the people of Vienna after yet another vile act of terrorism in Europe".
"These evil attacks against innocent people must stop. The US stands with Austria, France, and all of Europe in the fight against terrorists, including radical Islamic terrorists," he wrote.
Democratic presidential candidate Joe Biden condemned what he called a "horrific terrorist attack," adding, "We must all stand united against hate and violence."
German chancellor Angela Merkel expressed "solidarity with our Austrian friends", while European Council President Charles Miche strongly condemned "this cowardly act that violates life and our human values ... We stand with Austria."
"I am deeply shocked by the terrible attacks in Vienna tonight. The UK’s thoughts are with the people of Austria - we stand united with you against terror," said British Prime Minister Boris Johnson.
Meanwhile, India Prime Minister Narendra Modi and Australian Prime Minister Scott Morrison expressed shock at the "dastardly terror attacks".
Canadian Prime Minister Justin Trudeau described the attack as "horrific and heartbreaking".
"We condemn in the strongest terms possible this act of terrorism," he added.
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