Police were called at about 12.15am on Wednesday to reports of a fight in progress in Burket Close, Southall.
A Metropolitan Police spokesman said officers and London Ambulance Service attended and found a 17-year-old male suffering injuries.
They said despite the best efforts of the emergency services he was pronounced dead at the scene.
His next of kin have been informed. Four males have been arrested on suspicion of murder. Police have yet to release their ages.
A witness described how the teenager was savagely beaten and stabbed after two groups clashed in the street.
The local, who did not want to be named, intervened at one stage to try and stop the violence telling the group “you are going to kill him.”
He told the Standard: Some guys turned up in a car and they started fighting with people on the street. There was screaming and fighting. Someone shouted ‘they attacked my family’. The fighting was outside a house on Burket Close it was utter mayhem.
