At least three people have been injured in a knife attack on the streets of Vienna, with the attacker possibly injuring a fourth nearby.

Vienna police said the first attack happened at about 7:45 pm on Praterstrasse in the heart of Vienna.

"According to eyewitnesses a man has attacked three persons - a man, his wife and their adult daughter," police spokesman Patrick Maierhofer said..

"The three persons suffered heavy to life-threatening injuries and are currently in a hospital."

A fourth person was attacked with a knife nearby shortly afterward, and police said a link between the attacks "is part of ongoing investigation".

Police are still searching for the suspect, and a motive has not been established.

A forensic officer at the scene of the attack. Credit: PA

'He had lost all control'

Witness Silvia Franke said she was about 100 metres away when the first incident happened.

"I saw how someone in the middle of the street was screaming and shouting - I thought he was beating his dog judging from his hand movements," she said.

"And in reality he was stabbing this man."

"He could have stabbed me as well had I been there by chance," Franke said.

"He had lost all control."