Videos circulated on social media of a gunman running down a cobblestone street shooting and shouting. One showed a man gunning down a person outside what appeared to be a bar on the street housing the synagogue.
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 attacks.
U.S. 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 U.S. stands with Austria, France, and all of Europe in the fight against terrorists, including radical Islamic terrorists."
Democratic presidential candidate Joe Biden condemned what he called a "horrific terrorist attack," adding, "We must all stand united against hate and violence."