Screaming and shouting was heard on an estate before an 18-year-old man was found with fatal stab wounds, a resident said.
The teenager was given CPR by a member of the public on the estate in Greenwich, south-east London, but died before emergency services arrived, a witness to the aftermath of the stabbing said on Tuesday.
“There was a bike and car that I saw speeding away after all the noise,” the witness added.
Another resident said: “I heard some screaming and shouting and thought I heard some dogs barking, but basically that is what I hear every single day.”
Detective Chief Inspector Richard Leonard, of the Metropolitan Police’s homicide and major crime command, said: “We know that several people reported seeing the victim in a confrontation with three other males on mopeds before an altercation in which he sustained his injuries.
“The males then rode away from the scene. I need anyone who may have information that could assist us to make contact as quickly as possible.”
A crime scene is still in place and forensics officers are examining the area.
Formal identification of the victim has not yet taken place and a post-mortem examination will be held in due course.
No arrests have been made.
Anyone with information is asked to contact police on 101 quoting CAD 7560/22 October, or alternatively contact Crimestoppers anonymously on 0800 555 111.