Police were called shortly before 7:00 p.m. on Thursday with reports of a three-man fight in Parsons Green, Metropolitan Police said.
Officers attended the scene with colleagues from the London Ambulance Service, but a 19-year-old was pronounced dead. A second man, also in his late teens, was taken to hospital with non-fatal injuries.
The third man had left the scene before the police arrived. Parsons Green station and some surrounding roads have been closed and a crime scene has been declared.
A murder investigation has been opened and the police are investigating to locate the next of kin.
An autopsy will be performed in due course.
The Met said: “Homicide detectives have been notified and are now on the scene. Investigations are continuing. There were no arrests. “
A section 60 was put in place until 6 a.m. on Friday, covering postal codes SW3, SW6, SW10.
Local MP Greg Hands called the situation “truly horrible”, saying his thoughts are with the victim’s family and friends.
Charity director Robin Pritchard, 47, said he saw the victim lying on the ground in a “pool of blood” as someone gave him CPR.
“It was strangely calm and quiet. I think everyone was in shock,” he added.
Something awful has happened at Clapham South….someone on the floor lots of police & ambulance crew. This is the second awful accident in the last 2 weeks so sad 😱 police tape everywhere…— Katy (@kbsimmo15) November 2, 2018