The Prime Minister and the NSW Police Minister have praised bystanders who put themselves at risk by tackling a man armed with a knife in central Sydney and holding him down until police arrived.

Key points: Four men saw the commotion and chased the suspect down the street before knocking him down with a cafe chair and pinning him with a milk crate

Four men saw the commotion and chased the suspect down the street before knocking him down with a cafe chair and pinning him with a milk crate Three of the men are from Manchester, UK, and said they were defending their "new home"

Three of the men are from Manchester, UK, and said they were defending their "new home" NSW Police Minister congratulates firefighters who also confronted the knife-wielding man

Commissioner Mick Fuller said the men who tackled the man suspected of stabbing two women were "the highest order of heroes".

Armed with nothing more than chairs, a milk crate and crowbar, several men chased the man as he ran up Clarence Street and jumped on the bonnet and roof of a car.

Four of those men — Paul O'Shaughnessy, Luke O'Shaughnessy, Lee Cuthbert and Alex Roberts — said they instinctively sprung into action when they saw the commotion below the York Street recruitment office where they work.

The men used chairs and a milk crate to restrain the man. ( Twitter via AAP )

"[He] jumped on a car wielding a knife with blood on it and we were like, 'Wow, what's going on here'," Paul O'Shaughnessy said.

"So immediately, we just got the troops and said, 'Right, let's go and see if we can help'."

He said they ran down through throngs of terrified bystanders fleeing the scene and "just continued to chase the guy while other people were just going away".

"Because obviously, he's wielding a knife. I don't know whether it's an instinct thing or what, but we was like, 'Right, we've just got to try and restrain this guy from doing any more'."

Paul O'Shaughnessy speaks to police outside of Hotel CBD. ( AAP: Dean Lewins )

The man was allegedly waving a large knife and then jumped back down from the car roof and into the middle of the road, shouting as his pursuers continued to close in.

They eventually knocked him down with a cafe chair, and then used a milk crate to hold his head to the footpath as first firefighters then police arrived to restrain him.

Mr Cuthbert — from Manchester, UK, along with Paul and Luke O'Shaughnessy — said they were shaken after the incident but did not hesitate to defend their "new home".

A 21-year-old woman's body was found at an apartment complex near the scene and another woman, aged 41, was taken to hospital after allegedly being stabbed by the armed man.

The hotel where a woman was found by NSW Police with stab wounds. ( AAP: Dean Lewins )

'He had blood all over him'

Paul O'Shaughnessy said Luke, whose professional profile states he is a champion Thai boxer, was the "real hero".

"He pursued him the hardest and managed to get a grip of him with another man," he said.

"We just waited for the cops to come. He had blood all over him."

'Chair Chap' (right) was also praised on social media for intervening, but has not been formally identified. ( Twitter: @ayusha77 )

The group of friends, three of whom have come from countries terrorised by men wielding knives, said they had joked about how to handle an attack.

"We talked about it as a team, we said somebody might attack one day and imagine he was in the vicinity what would [we] do?" Paul O'Shaughnessy said.

"For us it was just go and help straight away, don't even think."

Prime Minister Scott Morrison commended "the brave actions of those who were present" and said his "thoughts are with all those who have been impacted by this violent attack".

Mert Ney was arrested after the alleged stabbing incident in Sydney's CBD. ( Twitter: David Vaux )

'This is how Sydneysiders respond'

NSW Police Minister David Elliot said the men had "acted well and truly beyond the call of any citizen".

"Can I also congratulate those firefighters … as having also gone beyond the call of duty," he said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 9 seconds 1 m 9 s NSW Police praised the people who apprehended the man.

Channel 7 cameraman Paul Walker, who was stuck in traffic in the area during the incident, told the ABC a group of firefighters sprung into action when the suspect jumped onto the bonnet of a car.

"Suddenly these firies who were in a fire truck a couple of vehicles in front jumped out," he said.

"They had axes in their hand and were yelling at him to stop, put his weapon down and he just ignored them."

Mr Elliot said he would make sure the firefighters got the appropriate accolades and he would "be ensuring everybody knows this is how Sydneysiders respond".