British police were holding a 19-year-old man on suspicion of murder yesterday after a central London stabbing spree that killed an American 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, thought to be in her 60s, and injured five others, had triggered fresh terror fears. But authorities said there was no sign the Norwegian man of Somali origin was radicalized or had terror motives, with the evidence so far pointing to the attack being sparked by mental health problems.

"We believe this was a spontaneous attack and the victims were selected at random," senior police officer Mark Rowley told reporters. "We have found no evidence of radicalization or anything that would suggest the man in our custody was motivated by terrorism."

The attack took place late Wednesday in Russell Square, a plush garden square in the city center close to attractions including the British Museum. Police responded quickly to reports of multiple stabbings and immobilized 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 American woman but pronounced her dead at the scene. Among those injured were U.S., British, Australian and Israeli nationals.

Three of the five have been discharged from hospital and the other two are not in a life-threatening condition.

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 armed and unarmed 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.

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."

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."