A friend of the family said: "Harry had gone to Primrose Hill with a few friends to see the fireworks.

"The police haven't told the family too much yet. We don't know if Harry knew the man who was arrested, and we aren't clear on what happened, if there was an argument or a fight."

"His mother is devastated, the whole family are."

"It was a strange, eerie scene and a damning indictment on the knife violence gripping London.

"This wasn't a small event, there must have been around 30,000 people on the hill at that time last night.

"It's always been busy up there on New Year's Eve but last night was as packed as I'd ever seen it.

"This young lad was murdered in clear view of people."

Detective Chief Inspector Geoff Grogan, who is leading the investigation, said: "My thoughts and sympathies are with Harry's family and friends at this difficult time. He was a young man with the rest of his life ahead of him.

"His family are understandably devastated and specially trained officers are supporting them as they come to terms with their loss.

"A dedicated team of detectives are working hard to establish the events that led to such a senseless act of violence. We are reviewing CCTV footage from the area and are carrying out a forensic examination of the scene.

"We know Harry was with a group of friends near the viewing platform on Primrose Hill where they had gathered to celebrate the New Year. It is here where we believe Harry became involved in an altercation with devastating consequences.

"Primrose Hill was very busy at the time and I believe there are people who witnessed the incident who have not yet spoken to police.

"It is vital that we build a clear picture of the moments leading up to, and immediately after, Harry's murder and I need anyone with information about the altercation, or the attack, to contact us immediately."