One of the boys stabbed in the Whitgift centre has a punctured kidney, police have revealed as they appeal for witnesses to the attack.
Detectives are asking for people with information on the double stabbing in the Whitgift Centre on Tuesday to come forward.
Police are yet to make any arrests in connection with the attack which left two teenagers needing hospital treatment.
Officers were called to the Whitgift Centre at 6pm on Tuesday to reports of a group of youths fighting.
Eye witnesses have said the fight broke out near Yoke Menswear on the second floor of the shopping centre.
London Ambulance Service and police officers arrived to find two boys, 16 and 17, suffering from stab wounds.
The 17-year-old suffered a stab wound to his lower back which punctured his kidney. He remains in a stable condition in hospital.
The other 16-year-old victim had a slash wound to his cheek and a stab wound to his upper arm. He has been released from hospital.
A 20-year-old worker, who offered first aid to the 17-year-old victim, said there was a lot of blood as members of the public tried to help the injured teen.
Detective Constable Jennifer Anderson from Croydon police said: "This incident took place in a busy shopping centre, we know that some members of the public witnessed the fight prior to the victims being stabbed.
“Some of those people have already come forward but we are keen to hear from anyone else who may have seen what happened to help us with our enquiries."
Any witnesses or anyone with any information that could assist police should call Croydon CID on 101 or call Crimestoppers anonymously on 0800 555 111.