Up to 10 people chased the 17-year-old 200 yards along Streatham High Road to Prentis Road in Streatham, where he was cornered outside a postal delivery office and knifed in the chest.
Police were called shortly after 4pm yesterday and found the teenager with a stab wound.
He was taken to a central London hospital, where he was pronounced dead at about 5pm.
One witness said: “I saw the gang brandishing machetes and other knives.
They were chasing a boy, they looked like they were hunting him down.
Postal workers came out and were fighting to save his life.”
Another witness said: “It was very vicious, there was lots of shouting. The next thing we knew, a boy was lying on the ground and there was lots of blood. The postal workers were real heroes for stepping up.”
The attack happened near the Little Flowers, Growing Beans and Springtime nurseries.
A worker at Growing Beans said: “Thankfully, the kids didn’t see anything but we had to arrange for them to be picked up at a different point because of the incident.”
Four men and three women have been arrested in connection with the killing.
Forensic science officers were today scouring the area for clues.
Police believe they know the identity of the victim and his next of kin have been informed.
The boy is the 16th teenager to be killed in London so far this year, one more than in 2020, and the capital is on course to overtake 2019’s total of 26.
Scotland Yard has launched 55 homicide investigations in 2021.
The latest killing follows two other violent deaths in the capital this week.
Taylor Cox, 19, died on Tuesday after being shot in the head in a “targeted hit” near a nursery and primary school in Hornsey Rise Gardens, Crouch Hill.
Nathaniel Eyewu-Ago, 35, also died on Tuesday, six days after being stabbed in a brawl in Barge Walk, Greenwich.
Anyone with information on the Streatham killing is asked to call police on 101 quoting CAD 5897/10JUNE.