The Metropolitan Police have launched a murder investigation after the death of the child in Hackney, east London.

Officers and the London Ambulance Service were called at around 10.50pm on Wednesday and found the four-year-old boy with knife injuries.

He was taken to hospital for treatment but died from his injuries.

Police detained the woman, 41, at the scene and she remains in custody.

The force said she was known to the boy and they are not looking for anyone else in connection with the death.

A neighbour told Sky News her friend saw a man walking away from the building just before police arrived. She said he had been living there until recently.

A large crack in the communal front door to the three-storey building showed where police had broken in before rushing upstairs to the top floor flat where they found the woman and the dying boy.

The neighbour, a hairdresser, said: "I had just gone out to get into my car and saw all the police activity going on. I saw them bring out the woman. She had blood all down her front. She was breathing heavily and seemed to be in shock. Some of us wanted to go over and comfort her, but police told us to keep away."

The neighbour said the injured boy was brought out more than an hour later and put into an ambulance. She said that two police officers, a man and a woman, who left the building afterwards were visibly distressed.

She said: "We're all shocked. It's very distressing. God knows what's gone on. The woman is a lovely, friendly neighbour who says hello and stops to chat.

"I'm told she had just split up with the boy's father, but I didn't know that. I had seen the father there recently.

"My youngest is friendly with the boy who has died. I can't tell him what's happened. I'll just say they moved away."

Detective Chief Superintendent James Conway said: "This is an extremely distressing incident which will understandably cause shock and disbelief amongst the local community and those who attended the scene.

"We are working with our partners, including Hackney Council and the school community, to ensure support is there for people who need it."

He added: "Specialist officers from our homicide team are progressing this investigation and continue to work to establish the circumstances of this young child's death, and the events that led up to it.

"This incident occurred within the home and, while there is no wider public safety risk, additional officers will be present in the area as we recognise the impact this will have on the community."