A knifeman has killed a woman and injured five others in a possible terrorist attack in central London.

A 19-year-old man has been arrested on suspicion of murder following the incident in Russell Square on Wednesday night.

The victim, an American woman in her 60s, was treated at the scene but died a short time later. Two people remain in hospital and three others have been discharged.

Police said the incident was possibly motivated by terrorism but said mental health was a "significant" factor.

Forensic officers at the scene on Wednesday night. Credit: PA Wire

Tourists from Australia told ITV News they saw the body of the victim as they walked up towards their hotel moments after the Russell Square stabbing attack.

Gary Griffiths, from Sydney, said: "We saw a dead person on the ground."

Witness Phillipa Bagley said the suspected attacker had a motorcycle helmet balanced on his head and was "walking up and down the street" as wounded victims lay in the road.

An eyewitness told the Mail Online he saw a woman, who he believes to be the victim who later died, bleeding from her back being comforted by her friend who was also injured.

"I heard a scream, and then we went to the park and I saw a girl lying on the floor with blood coming from her back. Another girl had blood on her arm," he said.

"I just saw blood just next to her. She was lying on the floor with a friend hugging her."

The victim was "slumped" against a metal fence opposite the Imperial Hotel as her friend comforted her and tried to stop her falling, he said.

He said he believed they were Spanish as two men they were with were shouting 'tio lo vi todo', meaning 'I saw everything'.

Mayor of London Sadiq Khan has urged the public to remain "calm and vigilant" as police continue to investigate whether the knife attack was terror-related.

Mr Khan is cutting short his holiday to return to the capital.

The attack comes less than 24 hours after it was announced that more armed police will patrol London's streets in the wake of the recent terror atrocities in Europe.

It also comes days after Metropolitan Police chief Sir Bernard Hogan-Howe warned that a terror attack in the UK was a case of "when, not if".

Witnesses described seeing armed police in the area following the incident.

Met Police Assistant Commissioner Mark Rowley said police were called just after 10.30pm "to reports of a man armed with a knife assaulting people at Russell Square".

He said: "Armed officers arrived at the scene within five minutes. They discharged a Taser while confronting and arresting the suspect.

"Six people have been injured, including the one woman who sadly died at the scene, and the others have received various injuries.

"Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry.

"Terrorism as a motivation remains but one line of inquiry for us to explore."

The suspect received treatment in hospital before being released and officially arrested on suspicion of murder.

He remains in custody.

A police forensic officer at work in Russell Square. Credit: PA
A body is removed from the scene in Russell Square. Credit: PA Wire
The incident happened in Russell Square in London. Credit: Google Earth

Mr Rowley added that Scotland Yard's homicide command is investigating the death, supported by the counter-terrorism unit.

Mr Rowley said their would be an increased police presence on the streets of London on Thursday, including armed officers, and asked the public to remain "calm" and "alert".

Witnesses or people with information can contact the incident room at Lewisham police station on 020 8721 4868 or if they wish to remain anonymous, Crimestoppers on 0800 555 111.