An American woman was killed and five others were injured in a stabbing spree by a 19-year-old Norwegian of Somali origin in central London, police authorities said.
British police said the attacker was arrested and there was no evidence the man was radicalised or motivated by terrorism.
The police said mental health was a "significant factor" in the wanton attack that took place in Russell Square, a busy area close to the British Museum where many hotels and restaurants popular with tourists are located.
According to the London Metropolitan Police, the young attacker was arrested at 10.39 p.m. on Wednesday night after an officer fired a taser gun at him, BBC reported.
He was taken to a south London police station after receiving treatment in hospital.
"While the investigation is not yet complete, all of the work we have done so far increasingly points to this tragic incident as having been triggered by mental health issues," Metropolitan Police Assistant Commissioner Mark Rowley said, speaking outside Scotland Yard headquarters, describing the situation as "spontaneous attack".
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The woman who fell to the stabbing attack was an American national, Rowley said, adding that those injured were Australian, American, Israeli and British nationals.
An eyewitness said the man was carrying a knife and had blood on his hands while another witness added he saw a "dark-skinned man" in his "late 20s" being arrested.
The authorities said there would be an increased police presence on the streets of London from Thursday "for reassurance and safety".
The attack was not linked with a separate stabbing in Wandsworth, southwest London, on Wednesday in which a man was killed and two others were injured, the Metropolitan Police said.
The stabbings came after authorities earlier on Wednesday announced London would increase its presence of armed police after recent terror attacks in Europe.
In the terror attacks of July 7, 2005, the Underground subway line running close to Russell Square was hit by the most devastating of the four bombs that killed 52 people and left injured over 700 others.
The police have not established a terror link to the attack.
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