London (AFP) - British police were holding a 19-year-old man on suspicion of murder Thursday after a central London stabbing spree that killed a US woman but appeared unrelated to terrorism.
Cities across Europe have been on edge after a string of attacks in recent weeks and the overnight rampage that killed the woman and injured five others, had triggered fresh terror fears.
But authorities said there was no sign the Norwegian man of Somali origin was radicalised or had jihadist motives, with the evidence so far pointing to the attack being sparked by mental health problems.
The US woman killed was named as 64-year-old Darlene Horton, whose family is receiving support from police.
US Secretary of State John Kerry described the attack as a "horrific mass stabbing" and said he grieved for the victims.
Senior police officer Mark Rowley said: "We believe this was a spontaneous attack and the victims were selected at random.
"We have found no evidence of radicalisation or anything that would suggest the man in our custody was motivated by terrorism."
The investigation "increasingly points to this tragic incident as having been triggered by mental health issues", he told reporters.
- 'Heartbreaking' -
The attack took place late Wednesday in Russell Square, a plush garden square in the city centre close to attractions including the British Museum.
Police responded quickly to reports of multiple stabbings and immobilised the suspect with an electric stun gun within six minutes.
However, he had already injured two women and three men. Paramedics battled to save the US woman but pronounced her dead at the scene.
Among those injured were US, British, Australian and Israeli nationals.
A British man remained in hospital on Thursday evening in a serious but stable condition, after suffering a stab wound to the stomach. The four others have been discharged from hospital.
An Australian woman was stabbed in the back while two men, one American and one Australian, each suffered a stab wound to the chest. An Israeli woman was stabbed in her bicep, police said.
Kerry praised the "courageous efforts" of first responders and sent his condolences to those impacted by the stabbings.
"I think all of us in the world have learned that violence only leads to more violence," he said.
"And at this difficult moment, the United States stands with our friends in Great Britain as the authorities seek the full facts as to exactly what happened, what the motive was, whether others may have been involved or not," he told reporters in Buenos Aires, Argentina.
US ambassador to Britain Matthew Barzun took to Twitter to send his condolences.
"Heartbreaking news that a US citizen was killed in #RussellSquare attack," he wrote. "My prayers are with all the victims and their loved ones."
With other European cities on edge following recent attacks, including those in Paris, Nice and Brussels, London Mayor Sadiq Khan appealed for calm and vigilance.
"We all have a vital role to play as eyes and ears for our police and security services and in helping to ensure London is protected," he said.
Extra officers were being deployed on the streets in a bid to reassure the public.
Police had announced hours before the stabbings that the first of an additional 600 armed officers were trained and ready to hit the streets.
British police officers are not routinely armed but the decision to boost the number of armed officers in London from 2,200 was taken after last year's Paris attacks that killed 130 people.
- 'I saw the body' -
Xavery Richert, 22, a French tourist staying in a youth hostel on the Russell Square said he saw a man chasing a screaming woman down the road.
"I thought it was a bag snatching... she was not hurt," he told AFP.
"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 added: "It's such a safe area and very quiet especially at night -- why would somebody commit an attack in such a quiet area?"
Police were interviewing the suspect, his family and other witnesses and authorities were conducting a full intelligence review.
Ida Dahl Nilssen, spokeswoman for the Norwegian criminal police, told AFP that they had been informed of the arrest.
"He is recorded as having been outside Norway since 2002, that is to say, a long time ago given that he is only 19," she said.
Since August 2014, the terror threat level in Britain has been classed as "severe" -- the second highest level, meaning an attack is considered "highly likely".
Wednesday's stabbings came two days after a mentally ill Somali-born man was jailed for life for a knife attack at a London Underground station.
Paranoid schizophrenic Muhaydin Mire, 30, tried to behead a commuter in December in an Islamic State-inspired attack.