A woman has been beheaded in a back garden after being attacked with what is thought to be a machete.
The victim was found at an address in Nightingale Road, Edmonton, north London, just after 1pm yesterday.
Scotland Yard gave few details but said that a man had been arrested.
Eyewitnesses reported seeing a man with a machete in the area.
There is no suggestion that the killing had any terrorist motive.
"Police were called to an address in Nightingale Road at 13.07pm today to reports of a stabbing incident," a spokesman for Scotland Yard said.
"On arrival officers discovered a female in a garden area.
"A man has been detained and remains in custody.
"At this very early stage we are not in a position to give further details."
Scotland Yard said a firearms officer had suffered a broken wrist in the course of arresting the 25-year-old man who is being held on suspicion of murder.
Det Chief Insp John Sandlin, leading the investigation, said: "This was a highly-visible attack in broad daylight on a residential street. I can understand why this may cause people concern, however, we are confident that we are not looking for anyone else at this stage.
"While it is too early to speculate on what the motive behind this attack was, I am confident, based on the information currently available to me, that it is not terrorist-related.
"Local officers and specialist detectives are at the scene and will remain there for many hours to come.
"Specially-trained family liaison officers will be deployed to support the family. We request that their privacy is respected at this very difficult time."