A 15-year-old boy has been arrested on suspicion of murdering Harry Pitman, who was fatally stabbed on New Year's Eve.
Harry, 16, from Haringey, was attacked in Primrose Hill, north London, at about 23:40 GMT on Sunday.
The boy was arrested - along with an 18-year-old man on suspicion of affray - on Tuesday night, the Met said.
Specialist detectives have found there was no indication the attack was racially motivated, the force added.
Det Ch Insp Geoff Grogan, who is leading the investigation, said despite having made the arrests, he was "still very keen to hear from anyone who has footage or information".
On Sunday, a 16-year-old boy was arrested at the scene on suspicion of murder and later released on bail pending further inquiries.
The 15-year-old boy and 18-year-old man arrested on Tuesday remain in custody in central London police stations, the Met said.
"Harry had been with a group of friends near the viewing platform on Primrose Hill where they had gathered to celebrate the new year and watch the firework display," Det Ch Insp Grogan said.
"It was there that Harry became involved in an altercation that would have devastating consequences."
Officers provided first aid before paramedics arrived, but despite the efforts of emergency services, Harry died at the scene shortly before midnight, police said.
Det Ch Insp Grogan added: "We are aware of speculation, especially on social media, that the attack may have been racially motivated and I can confirm that there is no indication of a racial motive."
On Tuesday, mourners, including Harry's mother, brother and sister held a vigil in Downhills Park, near the family home.
Tayla Pitman, Harry's older sister, had said the family was "devastated".
London's independent victims' commissioner, Claire Waxman, described Harry's death as "a senseless tragedy".
She added: "I'm aware that a group of residents has raised concerns with the Royal Parks about a rise in violence and anti-social behaviour in the Primrose Hill area over the last couple of years."
Ch Sup Andy Carter, leading policing in Camden, said he was "deeply saddened at the needless loss of another young life to knife crime".
"New Year's Eve at Primrose Hill is a celebratory event that everyone should have returned safely home from," he said.
"As a partnership, we take the safety of park users seriously, and we will continue to work with Camden Council and Royal Parks to address local issues."
He said there would continue to be a "heightened police presence" in the Primrose Hill area in the coming days.
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