A woman in her 20s has died in Hanwell, west London.
Emergency services were called just after midday to reports of a seriously injured woman in Uxbridge Road.
Officers, paramedics and London's Air Ambulance attended, but the woman was pronounced dead at the scene at 12:50, the Metropolitan Police said.
Police said they are treating her death as unexplained but added that a 28-year-old man was arrested at the scene on suspicion of murder.
He has been taken to a north London police station where he remains in custody.