Police did not give any further information about the incident. (Representational Image)

A woman was taken to hospital after being stabbed in downtown Sydney this morning in what appeared to be an unprovoked attack by a man who was restrained by members of the public and then taken into custody, police said.

The unidentified man had also tried unsuccessfully to stab other people, police said, in an incident that brought the central business district in Australia's largest city to a standstill in the early afternoon.

"We have a female person with a stab wound to the back currently being treated by paramedics and is en route to a Sydney hospital," Police Superintendent Gavin Wood told reporters in Sydney.

He said the woman's injuries were not life-threatening.

Wood also said the attack appeared to be unprovoked but police were keeping an open mind.

Police were also investigating whether the body of a woman found inside an apartment near where the stabbing took place was linked to the incident, a New South Wales state police spokeswoman said.

Video images on Twitter showed a young man running across a city intersection and jumping onto the hood of a car, waving what appeared to be a long-bladed knife.

The man, who was yelling, fell to the ground when the car moved and was confronted by a person holding a chair, the video showed. It was unclear what the man was calling out.

Other footage showed the man pinned to the ground by several people holding two chairs and a milk crate on top of him.

Australian Prime Minister Scott Morrison praised those who stopped the assailant.

"The attacker is now in police custody following the brave actions of those who were present at the scene and were able to ... restrain him," Morrison said.

A woman who answered the telephone at a Subway store near the scene told Reuters, "We saw the person with a knife over there bleeding.

"The cops are here and they blocked the road right now, so we can't go outside. They said, 'It's a crime scene, you can't come out,'" said the woman, who declined to give her name.

Police said the man was acting alone.