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My brother was the hero: stabbing witness

Hailed as "heroes of the highest order", bystanders bravely chased down a knife-wielding man on a stabbing rampage in Sydney's CBD before wrestling him to the ground and pinning him down with chairs and a milk crate.

The 21-year-old is accused of killing a young woman in a Clarence Street apartment on Tuesday before stabbing a 41-year-old woman at the Hotel CBD.

Detectives allege Mert Ney subsequently tried to stab countless others as he wandered the streets.

Witness Paul O'Shaughnessy said his brother, 30-year-old Luke, first saw the knife-wielding man covered in blood jump onto a car and start yelling after 2pm.

"We were like 'Wow, what's going on here'," the clearly-shaken 37-year-old told reporters.

The pair, along with several colleagues, raced outside and chased the man down.

"For us, it was just about restraining him," Paul O'Shaughnessy said.

"My brother, he was the hero. He got a grip of him, along with another guy we don't know, and put a crate on his head. He was just mumbling religious things."

Mr O'Shaughnessy said he was "very rattled" but proud of his brother and the other man who helped.

"There was a very big crowd but they were the ones who restrained the guy and just got a grip so he didn't do any more damage," he said.

"We managed to wait until the police came and they did a fantastic job. My brother continued to keep him down just in case, which is very brave."

Video footage from the scene shows a number of men in pursuit of the alleged knifeman.

One of those men, Jamie Ingram, said he didn't think of his own safety.

"I followed him up York Street trying to get him to focus on me rather than anyone else," he told the Nine Network.

"There are lots of pedestrians there, and I didn't want him to sort of lash out at them. I wasn't scared, just wasn't really focused on that. I was just focused on what I was doing."

NSW Police Commissioner Mick Fuller said the men were heroes "of the highest order".

"Three members of the public, who can only be described as heroes of the highest order, have engaged this 21-year-old man and have placed him essentially under arrest even though he was brandishing a large butchers knife and was clearly dangerous," he told reporters on Tuesday night.

Superintendent Gavin Wood said the men's intervention had stopped other people from being hurt and described them as "significantly brave".

"These people are heroes," he told reporters.

"These members of the public have jumped into a situation which was extremely dangerous, extremely hostile and they brought a person who we will be alleging stabbed an innocent person for no specific reason into custody and allowed us to do our job."

Numerous videos of the man's rampage have emerged on social media including footage showing a bystander holding a chair confronting the man who was yelling: "Shoot me, f***ing shoot me in the f***ing head."

One man can be heard screaming at Ney: "You f***ing piece of shit, do you know how many people you just stabbed? You stabbed a chick."

Seven Network cameraman Paul Walker - who filmed Ney being detained - said the man was "full of blood".

"The guy with the knife just kept running and stopping and running and stopping," he told reporters.

"Someone grabbed a milk crate and smacked it over his head."

Witness David Vaux said events unfolded very quickly.

"He was held very securely and I don't think he was in a state to basically breathe," he told reporters.

"It's an extraordinary response by the public and just demonstrates that people are prepared to do the right thing if called upon."

Ney was in police custody in hospital on Tuesday night. The 41-year-old woman was taken to hospital in a stable condition with a stab wound to her back.

© AAP 2019