One of the victims, a woman in her 60s was treated at the scene but was pronounced dead a short time later.
A 19-year-old man was arrested at 22:39 after a Taser was discharged by an officer. The Met said mental health was a “significant factor” in the events.
Scotland Yard’s 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, Police said they were keeping an open mind over the motive, 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, according to BBC reporter Andy Moore at the scene.
The condition of those injured and the extent of their injuries is not known at present. (Courtesy BBC News)