Police were called at 4.15pm on Monday, 20 March to a report of a stabbing on Elmwood Road, Croydon.
Officers attended along with the London Ambulance Service.
A man, believed to be aged 18, was found with a stab injury. He was taken to the hospital in a critical but stable condition.
One man was arrested nearby on suspicion of grievous bodily harm. A crime scene remains in place.