The terror threat level in Britain is currently set at severe -- the second highest ranking -- meaning an attack is "highly likely".
A woman was killed and five people injured in a knife attack in central London on Wednesday which police said they are investigating for possible terrorist links.
A 19-year-old man, with suspected mental health issues, was arrested in Russell Square in the city centre where the stabbing took place.
A woman in her 60s was treated by paramedics at the scene, near the British Museum and University of London, but pronounced dead. No details were released about the condition of the other victims.
The arrested man is currently in police custody in hospital.
A man has been detained and arrested by police #RussellSquare https://t.co/jsPqqqeOJa— Metropolitan Police (@metpoliceuk) August 4, 2016
"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 but one line of inquiry for us to explore," Police Assistant Commissioner Mark Rowley said in a statement.
Officers were called to Russell Square at 10.33pm local time (2133 GMT) following reports a man armed with a knife was attacking people.
The man was arrested six minutes later as a Taser electroshock gun was fired by officers.
The police cordoned off the site, a busy tourist area with a string of high end hotels.
Officers are currently on scene at #RussellSquare. Up to six people injured and one person has died— Metropolitan Police (@metpoliceuk) August 4, 2016
Xavery Richert, 22, a French tourist staying in youth hostel on the square, told AFP: "I was buying a beer when I heard a woman shouting who was being chased by a man.
"I thought it was a bag snatching... she was not hurt. I came out for a cigarette, I went back, there were firefighters, police, and then I saw the body under a sheet. You could only see the feet sticking out."
Russell Square resident Constantine Somerville said: "It's such a safe area and very quiet especially at night -- why would somebody commit an attack in such a quiet area?"
If a terror link is found, it would be the latest in a string of recent attacks in Europe.
Last month a driver ploughed a truck through a crowd leaving a fireworks display in Nice, killing 84, while two men slashed a priest's throat in northern France. Both attacks were claimed by the terrorist group DAESH.
The terror threat level in Britain is currently set at severe -- the second highest ranking -- meaning an attack is "highly likely" and earlier on Wednesday London's police announced it would boost the number of armed officers on the streets.