A 20-year-old man was stabbed to death in the street after asking a drug dealer to move on, police said.
Omid Saidy was killed outside Parsons Green Tube station, where 30 people were injured in a terror attack last month.
Scotland Yard said the victim confronted a drug dealer and another male who was with him, before chasing the pair.
But he was fatally stabbed when he caught up to them and Mr Saidy, from Fulham, was pronounced dead at the scene at 8.30pm on Monday.
Detective Chief Inspector Noel McHugh said: "A young man has tragically lost his life for simply asking a drug dealer to move on. I urge anyone who can assist our investigation to come forward without delay."
The victim's 20-year-old friend, who arrived on the scene on a moped, was also stabbed when he came to his aid, the Metropolitan Police said.
The bike collided with a parked vehicle at the scene, but Scotland Yard are not treating the incident is not a "moped-enabled crime."
A 16-year-old white male, who was discharged from hospital after he was treated for his injuries, has been arrested on suspicion of murder and attempted murder.
A second suspect, described as a black male dressed in dark clothing, fled down Harbled Road in the direction of Fulham Court.
Appealing for witnesses, Mr McHugh added: "It is imperative that we speak to anyone who was in the area of Parsons Green station between 7pm and 8pm who may have witnessed the murder or the actions of the suspects immediately following the incident.
"In addition, I am keen to identify the black male who fled Parsons Green Lane immediately following the murder and am appealing for footage that members of the public may have recorded as events in the area took place."
Meanwhile, a 16-year-old boy has been charged with murder after a 20-year-old man was stabbed to death in Forest Gate in east London on Sunday afternoon.
Police said a postmortem found the victim, who has not been formally identified, died from a stab wound to the heart.
The 16-year-old is due to appear at Barkingside youth court on Wednesday.