A 20-year-old man has been arrested on suspicion of the murder of a teenager believed to have been attacked by a gang wielding swords and knives.
Hani Abou El Kheir, 16, died from multiple stab wounds after he was set upon on the edge of Pimlico, one of London's most affluent neighbourhoods, on Sunday evening in a suspected drugs-related attack.
The suspect was arrested in north London during an early morning raid, Scotland Yard said.
Police believe at least 10 people were involved in the incident.
The teenager, who lived on the nearby Churchill Gardens estate, was taken to King's College Hospital but died later.
Detectives said they are keeping an "open mind" regarding a motive and are being assisted by a number of witnesses who have already come forward.
Detective Chief Inspector Neil Attewell, of the Homicide and Serious Crime Command, said Hani suffered a "horrific and sustained vicious attack".
He added: "The tragic events of last Sunday have left Hani's mother and wider family completely devastated.
"Hani's mother has lost her only child and is still coming to terms with the heartbreaking events that have led to the death of her son.
"I am directly appealing to the parents and carers of those believed to be involved - if you have slightest indication that your child may have been involved in this incident I encourage you to get them to hand themselves in to the police. Failing to do so will result in my officers pursuing them until we have caught all those responsible."
A note from the victim's mother, Pauline Hickey, was left with flowers at the scene where he died.
It read: "My baby. My heart is broken. I love you so much, the pain is unbearable. Your loving mother."
Another tribute attached to a nearby bus stop was a copy of a touching note written by Hani to his mother, alongside a picture of him as a child.
It said: "Thank you for being the best mum. Love to mummy, thank you for everything. Love from Hani."
The tribute was signed by relatives including the teenager's grandparents, and cousins in Ireland.