Devastated coaches paid tribute to young boxer Teon Campbell-Pitter, from Lewisham, as they said he had “his whole life ahead of him” until he was attacked.
“This simply cannot keep happening to young people,” they added.
Witnesses spotted a gang roaming around a park before “all hell broke loose” and suspects fled in all directions.
Police were called shortly after 4pm to reports of a brawl outside the Moonshot Center in Angus Street, New Cross on Wednesday.
Despite the efforts of paramedics and the air ambulance, the teenager was pronounced dead from stab injuries at the scene.
On Thursday, Scotland Yard said Teon was attacked in “broad daylight” in a “busy area” and urged witnesses to get in touch.
“I would like to thank those who have already come forward and spoken to police about what they saw but I know there will be others who have yet to do so,” said DCI Nigel Penney, who is leading the murder investigation.
“I would urge you to make contact and provide us with any information you have, no matter how insignificant it may seem. We are building a picture of the events that led to Teon’s murder, and your information could prove to be a vital piece of that picture.
“I would also appeal to anyone who took pictures or filmed the events around the time of this attack; again – please get in touch with us and share what you have.”
He said information shared with police would be treated in “strictest confidence” but Crimestoppers could also be contacted completely anonymously.
A cordon remains in place close to Deptford Green School and the Double Jab Boxing Club.
In a tribute, the Double Jab Boxing Club said: “It is with a heavy heart that we have to announce the death of one of our junior members who was fatally stabbed this evening outside our gym.
“A 16-year-old boy with his whole life ahead of him taken too soon by knife crime.It is these devastating moments which highlight how urgent our fight against knife crime is.
“This simply cannot keep happening to young people. Our hearts are with the family of the young man, who are now facing the loss of their beloved son. We are devastated.”
Police have pledged extra officers in the area after the “senseless attack” as they said they are aware of “the concern and shock” in the community.
“I want to reassure people that a team of experienced detectives are investigating Teon’s murder and they will do everything within their power to identify and arrest those responsible,” said Detective Superintendent Danny Gosling, of the Met’s South East Command Unit.
“But they need your help and I would appeal to those who have information to do the right thing by Teon’s family and friends and tell police what you know.
“Residents in the New Cross area will see an increased police presence in the coming days, including local officers. If you have any concerns, or have information that could assist, please approach them and speak to them.”
A dog walker, 41, who stumbled upon the fight, told the Standard: “It was carnage, all hell broke loose. I had spotted some boys hanging around near the community center and hear there was a bad atmosphere.
“I can’t put my finger on it but there was tension. The next minute, a boy was bleeding on the ground, others were scattering in all directions.
“There was shouting and people trying to help the injured lad. The police were here quickly and moved us away from the scene. Then an air ambulance cam. It was very distressing.”
A Section 60 granting police additional stop and search powers has been authorized for the whole borough of Lewisham.
It is the fourth teenage homicide in the capital this year, after a record 30 last year.
The previous high was 29 in 2008.
Donovan Allen, 18, stabbed to death at a block of flats in Enfield on February 7 was the year’s first victim.
Schoolboy boxer Tyler Hurley, 16, was fatally knifed on a 173 bus in Chadwell Heath on March 14.
City University student Sabita Thanwani, 19, was discovered with severe neck injuries in her Clerkenwell halls of residence five days later.
Suspects have been charged with murder in the three previous teenage killings.
There were 11 teenage homicides by April 14 last year.
Anyone with information that could assist police with solving Teon’s murder is asked to contact the incident room on 020 8721 4622 or tweet @MetCC.
Alternatively, you can leave information with the independent charity Crimestoppers on 0800 555 111 or at www.crimestoppers.org.