A 19-year-old man is under guard in hospital after a woman was killed and five people were injured in a London knife attack.
The woman, who has not been identified but is believed to have been in her 60s, died and two women and three men received "various injuries" in the attack in Russell Square late on Wednesday.
Police said they are keeping an open mind over the motive behind the attack, understood to have taken place near the Imperial Hotel, and said terrorism was "one line of inquiry that we should explore".
The killing is currently being investigated by the Met's homicide team with the support of the force's Counter Terrorism Command.
However Scotland Yard said early indications suggested "mental health is a significant factor in this case".
The suspect, whose identity has not been released, was arrested after officers fired a Taser.
As a result of the attack armed police and other officers will be deployed on the streets of London on Thursday as a "precautionary measure" to provide reassurance, the Met said as it urged the public to remain "calm, vigilant and alert".
Speaking outside Scotland Yard in the early hours of Thursday morning, Assistant Commissioner Mark Rowley said it had been a "tragic incident".
He said: "Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry.
"But of course at this stage we should keep an open mind regarding motive and, consequently, terrorism as a motivation remains one line of enquiry for us to explore."
Police were called to Russell Square, a popular tourist area near the British Museum in Bloomsbury, shortly after 10.30pm.
Rowley said armed officers arrived at the scene within five minutes, with the suspected attacker arrested shortly after 10.39pm. He is currently in police custody at hospital.
Paramedics gave the woman "extensive treatment" at the scene but she could not be saved. The five injured people were all taken to hospital for treatment, the LAS said.
A private ambulance was seen removing the body at around 5.20am, after forensics officers had finished scouring the scene for evidence.
Witnesses described seeing armed police flood the area in the wake of the attack and officers established a large cordon to seal off the square where a forensics tent was later seen.
A heavy police presence and cordon remained in place on Thursday morning, with a forensics tent marking the spot where the woman was killed.
Businesses surrounding Russell Square remained closed, with normally busy coffee shops shut to customers as the investigations continued.
Scotland Yard appealed for witnesses to last night's incident with information to call police on 101 or call Crimestoppers anonymously on 0800 555 111.