Police were called to Leigham Court Road, SW16, at 3.23pm on Wednesday, 25 November, to reports of a man attacking people with a pole.
Officers attended along with paramedics from London Ambulance Service (LAS).
A 13-year-old boy was found with a head injury. He was treated at the scene before being taken to hospital. His condition has now been assessed as non-life-threatening and non-life changing.
A 35-year-old man was arrested nearby on suspicion of causing grievous bodily harm [GBH]. He was taken into custody where he remains.
The crime scene has now been closed.
Superintendent Ian Howells, Neighbourhood Lead for Lambeth and Southwark, said: “This was a shocking incident and has caused alarm and distress among the local community.
“We are supporting the family of the boy who was injured and have a number of officers in the area speaking with residents and also pupils from the school nearby who witnessed what happened.
“We would like to reassure the public that this was an isolated and contained incident.
“A man has been arrested and a thorough investigation into how the teenager came to be injured is underway.
“We are continuing to work with the school, local authority and wider partners and community to ensure the school and community receive the necessary support following this incident.”
Anyone with information or dashcam footage which could assist the investigation is asked to contact police via 101 quoting CAD4356/25Nov.