The controversial far-right group, Identitäre Bewegung (Identity Movement), was behind the action, which they described as a protest against terrorism.
The men and women stood directly before the St Stephen's Cathedral, wearing hoods and holding a black placard. Suddenly two of the men grabbed a knife and held it to the throat of a young woman and a man. Seconds later, both fell to the ground.
Passersby were horrified and police quickly brought the drama to an end.
The group's Facebook page stated that the protest was intended to draw attention to the issues of "mass immigration, Islamization and the threat of terrorism."
Police spokesman Paul Eidenberger has said the stunt was legal.
"The event was registered with the authorities as a act which would involve 15 people. Legally therefore, there are no concerns."