The knife attack took place near or at Nestroyplatz metro station earlier this evening, with the area now on lockdown.
Photos taken at the scene show blood on the floor and a heavy police presence. Residents have been urged to remain indoors.
Officials said at least three people had been “seriously injured”, all aged between 40 and 50. Two women and a man have been taken to hospital in a critical condition.
A specialist, heavily-armed police unit has been called to the scene in the heart of the city.
Trains are no longer stopping at Nestroyplatz station.
Witnesses said the knife attack may have been preceeded by a fight.
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