A woman was killed and five people injured in a knife attack in central London Wednesday which police said they are investigating for possible terrorist links.
A 19-year-old man was arrested in Russell Square, in the city centre, which was cordoned off after the attack as police swarmed the area.
A 60-year-old woman was treated by paramedics at the scene but pronounced dead. Two women and three men were also injured but no details have been released about their condition.
The arrested man is currently in police custody in hospital.
"Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry," London police's counter terror chief Mark Rowley said in a statement.
"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," added Rowley.
Police were called to Russell Square at 10:33 pm local time (2133 GMT) following reports a man armed with a knife was attacking people.
The man was arrested six minutes later with one of the officers firing a Taser electroshock gun.
Russell Square is a busy tourist area with a string of high end hotels and is also close to the British Museum and the University of London.
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 militant-inspired attacks in Europe.
Last month a driver ploughed a truck through a crowd leaving a fireworks display in the French resort town of Nice, killing 84, and two men slashed a priest's throat in the country's north. Both attacks were claimed by the Islamic State group.
In response to the recent attacks across Europe -- which include two in Germany also claimed by the Syria-based militants -- London's police announced Wednesday that it would boost the number of armed officers on the capital's streets. Police in Britain do not usually carry firearms.
The terror threat level in Britain has been set to "severe" -- the second highest ranking -- since August 2014, meaning an attack is "highly likely".
On Sunday, the head of London's Metropolitan Police, Bernard Hogan-Howe, wrote in the Mail on Sunday newspaper that it was a question of "when, not if" an attack happened in the capital.
In the July 7, 2005 terror attacks in London, one of the suicide bombs which hit the city's public transport system was detonated on the London Underground between Russell Square and King's Cross.
While 7/7, which killed 52 people, was the last large-scale terrorist attack in Britain, there have been a string of smaller incidents since.
A paranoid schizophrenic man was sentenced to life behind bars Monday for trying to behead a commuter at a London Underground station in an Islamic State-inspired attack.
Soldier Lee Rigby was killed in south London in 2013 by two British Muslim converts in a terrorist murder.