A schoolboy who was fatally stabbed in south-east London pleaded “I’m 15, don’t let me die” as he lay bleeding, a woman who held him as he lay on the ground has said.

The witness said she tried to save the boy after finding him wounded in the road outside her flat. Police were called at about 6.30pm on Sunday to reports of a disturbance in Woolwich. The boy was found at the scene with a stab wound and died a short time later.

The witness, who asked not to be named and has lived in the area for 14 years, said she was in her bedroom when a person across the road screamed: “Someone’s been stabbed.”

She said she grabbed a sheet and ran outside in bare feet to find the boy face down on the floor. He had a gash on his head and a “massive pool of blood” by his leg, and she tried to stem the flow of blood until paramedics arrived, she said.

“I asked if I could go and shower because I was covered in his blood, and they said yes so I went in and showered. I come back out and [the police] called me over because I was the last one with him. He was going to me: ‘I’m 15, I’m 15, don’t let me die,’ and I said to him: ‘You’re not going to die, mate.’”

She said the victim was “the most polite boy” when she had encountered him around their neighbourhood. Asked if she knew him well, she said: “No, I didn’t, but I always said hello and that, and he was always polite or moved out of the way when you were walking.

“The other day, he was sitting on the wall over there and he’s the most polite boy, he hasn’t got attitude. They were all dancing in the street the other day to a TikTok song.”

Ch Supt Trevor Lawry, of the Met police, speaking outside Plumstead police station on Monday afternoon, confirmed that the victim was 15.

He said: “Once again, we’ve had to tell a child’s family that their loved one has been killed in an act of violence using knives. Our thoughts are with them as they struggle to comprehend what has happened.

“The fact a 15-year-old teenager who had his whole life ahead of him has been taken from his family in this way is a stark and sobering reminder of the danger of zombie-style knives. We are committed to doing everything in our power to taking these weapons off the streets.

“We know that this murder will send shock waves throughout the community, and I understand the genuine concern this will create – we share those concerns.”

A spokesperson for the London mayor, Sadiq Khan, said: “The mayor’s thoughts are with the family and friends of the teenage boy who has been killed in Woolwich, as well as the wider community. This heartbreaking violence has no place in our streets. The mayor is in close contact with police leaders and there will be increased patrols in the local area.”

The boy’s next of kin have been informed. No arrests have been made and a crime scene cordon is in place. Anyone with information is asked to call police via 101 quoting reference CAD 5989/22Sep.