Witnesses have described “gruesome” scenes — with a pool of blood on the floor — after two people were stabbed in a  knife brawl at an Adelaide suburban shopping centre.

A 16-year-old boy had life-threatening injuries after he was stabbed in the leg in the Elizabeth City Centre, in Adelaide’s north, on Friday.

The teenager was taken to the Royal Adelaide Hospital.

Another 20-year-old man was stabbed in the abdomen during the brawl involving four people, and is reportedly in a stable condition.

Police were quickly on the scene after a fight broke out about 2pm and a man was arrested, with parts of the shopping complex immediately locked down.

The whereabouts of a fourth person were unknown, but he was not believed to be involved in the stabbing or in the possession of a weapon.

Police quickly confirmed that the members of the group were known to each other and the incident was not a terrorist attack.

Distressing video shown of the incident by Channel Seven showed a young man bleeding from a leg wound in a mall crowded with Black Friday shoppers before it was evacuated and businesses closed.

Another young man appeared to try to support his friend, only to slip in a pool of blood.

A shopper, Sajjad Alyasiri, told Seven News the scene was “pretty gruesome”.

“We just heard a whole bunch of screaming,” he said.

“We went outside and see two blokes, one on the ground with a bunch of blood around him and saw another guy with a knife in his leg.

“The knife looked pretty sharp, it was a lot of blood coming out … a lot of screaming.”

Superintendent Amin Sareen said it was a frightening incident for witnesses but reassured the public that it was not a random attack.

“The group was known to each other, police responded quickly, we have got the suspect, we have got the weapon that was used,” he said.

New laws banning the sale of knives to minors came into effect in South Australia on July 1.

Police could not confirm if there had been a metal detector patrol at the shopping centre.

“The metal detecting powers are definitely working … we do proactively go and use those powers in the shopping centre,” he said.

“I’m not sure if there was a patrol doing and exercising those powers at the time but I can tell you that police responded very quickly, hence we apprehended the suspect on scene.”

Police urged anyone with further information to call CrimeStoppers on 1800 333 000.

-with AAP