The incident happened in Peckham Library in Peckham Hill Street just before 6.40pm yesterday (November 5).
Officers and London Ambulance Service crews attended and found a man who had been shot.
He was taken to hospital where his condition was assessed as serious but not life-threatening.
No arrests have been made as police enquiries continue.
A spokesperson for the Metropolitan Police said: “A crime scene remains in place.
“Anyone with information is asked to contact police on 101 or online, quoting 6590/5NOV.”
A spokesperson for London Ambulance Service said: “We were called at 6.38pm on Wednesday 5 November to reports of a shooting on Peckham Hill Street, SE15.
“We sent resources, including an ambulance crew, an advanced paramedic, an incident response officer and a paramedic in a fast response car.
“We also dispatched a trauma team in a car from London’s Air Ambulance.
“We treated a man for a gunshot wound and took him a major trauma centre.”
