One woman was killed and five others injured by a man who went on the rampage with a knife in central London.

Armed police were called at 21.33 GMT on Wednesday after a man armed with a knife started to attack people in London's Russell Square, injuring several including one woman - a US citizen - in her sixties who later died. Police said it was a "spontaneous attack" and the victims, who also included British, American, Australian and Israeli people, were selected at random. None of those wounded have life threatening injuries.

The Metropolitan police assistant commissioner, Mark Rowley told reporters he believed the attack was a result of the suspect's mental health problems. He said the 19-year-old, who is Norwegian national of Somali origin, has been arrested but showed no sign of radicalisation. 

He emphasised there was no evidence the man was "in anyway motivated by terrorism".

Earlier Rowley had said mental health was a "significant factor in this case and that is one major line of inquiry".

But he added early on Thursday morning: "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, who arrived within five minutes of being called, used a Taser electric shock gun while detaining the 19-year-old suspect. He was taken to hospital but has since been discharged and formerly arrested.

The woman was treated at the scene but was pronounced dead a short time later. The other injured people - one woman and four men - are being treated. The southern part of the square has been cordoned off and a police forensics team is in place. 

The London mayor Sadiq Khan urged people to remain "calm and vigilant". He said his "heart goes out to the victims of the incident in Russell Square and their loved ones", adding police were doing a an "incredibly difficult job" and the safety of Londoners was his "number one priority". 

The threat level in Britain remains at "severe", its second highest level, meaning a strike was "highly likely". London police had already promised to deploy more armed offices after a spate of deadly attacks in other European countries.

"Londoners will wake up and in the morning they will notice an increased police presence on the streets, including armed officers," Rowley said.

"This is there to provide reassurance and safety. We ask the public to remain calm, vigilant and alert," he said.