A teenager who was stabbed to death in the street had been "running for his life" while covered in blood, an eyewitness has said.
The 18-year-old was knifed repeatedly on a residential road in Hayes, west London, on Monday afternoon while an onlooker screamed at his alleged attacker to stop.
The neighbour said she had seen the victim chased up Gledwood Gardens before he was allegedly set upon.
Despite the efforts of the emergency services to save the him, he died at the scene.
The witness, who did not want to be named, told the Press Association: "I thought it was just kids playing around. I wish I could have helped, I wish I could have done something.
"The white boy was running for his life, a young white teenager, he was stabbed multiple times.
"The boy had his shirt off and he was covered in blood.
"Then the boy kept on stabbing him and stabbing him and I walked to my wall and screamed 'stop it, leave him' and then I saw his hands going up and down and I was yelling 'stop it' while I was on the phone to 999."
The scene of the attack, at the junction of Gledwood Gardens and Gledwood Drive, was completely empty at the time, she added, but her screams brought residents out onto the street.
Despite the commotion, she said the attack carried on before the other man allegedly walked off with the knife still in his hands.
"There was nobody there and the boy just started to walk away, he didn't run, he didn't walk fast or anything - he just walked away.
"He did his job and then he walked away, he strolled away.
"By then, people had heard my screams and thought something happened to me and they came out, a woman was standing over the boy and other people had come to help."
She added: "I was also thinking as a mother. I did what I did as a motherly instinct as well, if the boy had run towards me I would have let him in my house, but I don't know if that would have been the right thing or not.
"You would do anything to protect someone when they're in trouble."
The victim was found shortly before 4pm by paramedics.
Another 18-year-old man, who police said knew the victim, was arrested nearby on suspicion of murder. He remains in custody in west London.
The Met Police said they believed the victim had been initially attacked at an address in Gledwood Gardens.
Detective Chief Inspector Noel McHugh said: "I would ask anyone who has information that could assist this investigation to contact the incident room.
"We are especially keen to hear from anyone who was in the area of Gledwood Gardens shortly before 4pm and witnessed the assault to come forward."