Police had to pull two men apart after the violent fight, which broke out outside a pharmacy in the CBD shopping complex, spilled onto the street about 1.30pm on Friday.

The 28-year-old victim was treated for upper body injuries and taken by ambulance to The Alfred hospital in a stable condition, an Ambulance Victoria spokeswoman said.

Two police officers pounced on the man's 40-year-old alleged attacker, pinning him to the ground.

"You stabbed someone," one officer says, in footage of the dramatic arrest aired by Channel 9.

"I stabbed someone my a--, I've done nothing wrong," the man responds.

It is understood the two men know each other.

Detective Senior Constable Andrew White said police officers, who were quick to the scene, had to pull the brawling men apart.

"There's been an altercation between two males outside a pharmacy earlier, that's spilled onto the tram stop in front of a few passersby," he said.

"We can tell you that one man has been taken to hospital with what appears to be a significant cut on his arm. Another is in custody at the moment.

"It is quite a shocking thing, obviously in the middle of Melbourne Central to be witness to such a significant fight between two males. And we appreciate the quick police response in taking control of the situation and getting both parties away from each other."

Senior Constable White said he could not discuss possible charges against the man in custody.

"We haven't gotten to that stage yet because we are still trying to work out what exactly has happened," he said.

A woman working at a florist next to the crime scene said she saw a man running through the corridor moments before police and an ambulance arrived.

"After a while security came and locked off the entrance, followed by quite a lot of police," the woman, who did not wish to be named, said.

Staff at the Shaver Shop, opposite the pharmacy where the stabbing happened, were very shaken.

"Immediately, you think what would have happened if that person came in here and ran towards you, with blood spitting out - you'd just s--t yourself," store manager Santo Pati said.

"I was with a customer and I saw trickles of blood in there," he said, pointing to a concrete pillar in the middle of the corridor.

"I just kept thinking, 'what if he comes in here?' It could have been one of us."

While the Shaver Shop workers did not witness the fight, he said, they heard screaming coming from the corridor.

"We're just glad we're safe and nothing's happened to us," Mr Pati said.

"Under police instruction we got told to close the shop, we didn't get advised how long it would take, so we went down to the other Shaver Shop in Lonsdale Street, to gather our thoughts."

Security guards closed the row of shops between Fossil and Mr Burger while police spoke to witnesses and examined the scene, but the area has since reopened.

The rest of the centre is functioning as normal.

Last month, a man was slashed across the face and stabbed in what police described as a random fight outside Melbourne Central.

That victim, aged in his 40s, suffered stab wounds to his arm and face and was taken to St Vincent's Hospital. A 42-year-old man was arrested at the scene.