A 13-year-old boy is among three teenagers who have been arrested on suspicion of murder after an 18-year-old was stabbed to death last night, police said.
Officers and paramedics went to Sydenham in south-east London at 8.30pm to reports that a fight had erupted between a group of youths.
They discovered the 18-year-old had been stabbed, and he was pronounced dead at the scene in Wells Park Road about an hour later.
His next of kin have been informed and a post-mortem examination and formal identification will take place in due course, Scotland Yard said.
A second youth, 17, was also stabbed and taken to a south London hospital where he remains in a stable condition.
Three teenage boys, a 13-year-old and two 15-year-olds, are being held by police on suspicion of murder.
A 17-year-old was also arrested on suspicion of assault.
The four are being held in custody at various south London police stations.
Detective chief inspector Russell Taylor of the Homicide and Serious Crime Command, who is leading the investigation, urged anyone with information about the attacks to contact police.
He said: "A number of youths gathered in Wells Park Road before a fight broke out. Many people would have witnessed the attack as well as the aftermath.
"I would urge those people to contact officers so that we can piece together the events that led up to this murder."
Asked if the attacks were thought to be gang related, a Scotland Yard spokesman said: "It remains a line of inquiry, but we are still at an early stage of the investigation."
Anyone with information should contact the incident room on 0208 721 4868 or Crimestoppers anonymously on 0800 555 111.