They said the 17-year-old was attacked by around ten men armed with knives, metal poles and baseball bats who drove to Burket Close, Southall, and knocked on his door at just after midnight on Wednesday.
An older relative was seen with blood on his t-shirt as another was placed onto a stretcher following the savage attack.
Sachin Puri, 29, who lives nearby, told The Independent: “There were a lot of cars revving up and down the road and then I saw two or three guys on the ground. The group were laying into him with knives and poles.
“I thought they were smashing people’s houses in at first. I couldn’t see their faces but I heard someone shout ‘Get the knives.’
“It was awful I heard the mother screaming ‘call the police.”
The victim’s older brother was heard wailing at police after the fatal ambush: “You didn’t do anything! What are you going to do? You only do something when we are dead.”
Four men - aged 21, 27, 31 and 71 - have been arrested on suspicion of murder, police said.
Det Supt Figo Forouzan, Head of CID in west London, said: “This is an extremely sad incident that will understandably cause concern in the community.
“I would like to reassure the community that we will carry out a thorough investigation to ensure those responsible are brought to justice.
“Whilst we have made arrests in this investigation, I would like to seek assistance from the public. I continue to appeal to anyone who has any information about how the events unfolded or anyone who may have captured the incident on their phone, dash cameras or doorbell footage to make contact with the police immediately.”
Anyone with information that could assist police is asked to call 101 or ‘X’ @MetCC and quote CAD 63/15Nov. You can also provide information anonymously to the independent charity Crimestoppers on 0800 555 111