A woman has died and five others were injured in a knife attack in Russell Square, central London.
Police and ambulance crews were called at 22:33 BST on Wednesday to reports of a man in possession of a knife and injuring people.
Up to six injured people were found at the scene; one woman was pronounced dead a short time later.
The man was arrested at 22:39; a taser was discharged by one of the arresting officers.
Terrorism is one possible motive being explored, the Metropolitan Police say.
Eyewitnesses have reported a police forensics tent has been erected on Southampton Row, outside the park in the centre of the square, which is close to the British Museum.
There is a heavy police presence in the area and the initial cordon around the crime scene has been extended, according to BBC reporter Andy Moore, who is at the scene.
The condition of those injured and the extent of their injuries is not known at present.