Officers were called to reports of a stabbing in Pound Lane, Willesden, at around 4pm on Saturday.
A 50-year-old woman was found with stab wounds and died at the scene.
A 30-year-old man – believed to have been known to the victim – has been arrested on suspicion of murder.
He has been taken into custody at a north London police station, the Metropolitan Police said.
Detective Chief Inspector Rebecca Woodsford said: “This incident happened in a busy part of Willesden, and I’d also like to thank the members of the public who provided first aid and called the emergency services.
“A suspect was arrested within several hours of the incident and he remains in custody.
“Our priority is to establish the circumstances surrounding what happened.
“However, based on our inquiries so far, there is nothing to suggest any wider risk to the public.”
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