A woman is dead and five people are injured after a knife attack in Russell Square in central London.
The attack happened about 10.30pm local time.
Police said a female victim - who was aged in her 60s - was treated at the scene, but died from her injuries.
A 19-year-old man was arrested after police used a taser to bring him down.
Police said the attacker had mental health issues.
Scotland Yard assistant commissioner for specialist operations, Mark Rowley, said the Met's homicide command was leading the investigation into the attack, with support from the counter-terrorism unit.
He stressed that police were keeping an open mind about the cause of the attack, and terrorism was "one line of inquiry" that would be explored.
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.
The condition of those injured and the extent of their injuries was not known.
The attack comes just hours after the Mayor of London announced that there would be a surge in the number of armed police on the city's streets, designed to reassure the public and deter attackers following terrorist attacks in Europe.
Police cordon around Russell Square eyewitnesses report armed police pic.twitter.com/TfLFXdJpRx— Raheem Kassam (@RaheemKassam) August 4, 2016