A woman has reportedly been beheaded in a back garden in Edmonton, north London.
Officers were called to the property off Nightingale Road at just after 1pm, in response to reports that a man was armed with a knife and had attacked a cat or a dog, ITV News reported.
After officers found a woman dead at the scene, they arrested a 25-year-old man on suspicion of murder.
In a statement, police said that a Taser was used during the arrest, with a firearms police officer believed to have sustained a broken wrist.
While the Metropolitan Police would not confirm the nature of the woman's injuries, several news agencies have reported that the woman was beheaded.
The woman's family have been alerted to the incident, police have confirmed.
DCI John Sandlin, leading the investigation, said: "This is 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.
"Whilst 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."
Police stressed that a "fast-moving and difficult situation" of this kind was "very very unusual."
Scene of fatal stabbing in Edmonton road pic.twitter.com/8LdjI0YRqK— Ruth McKee (@ruthamckee) September 4, 2014
Anyone with any information is asked to call the Incident Room on 020 8345 3865, or call Crimestoppers anonymously on 0800 555 111.
Additional reporting from PA