A woman has been stabbed to death in central London.

British police say the man who stabbed six people in London’s Russell Square was a Norwegian of Somali origin, but they have found no signs of radicalisation.

London Metropolitan Police Assistant Commissioner Mark Rowley says the investigation “increasingly points to mental health issues” as lying behind the rampage.

He said the woman who died was American, and the five injured people are British, American, Israeli and Australian. None had life-threatening injuries. Two remain in a hospital, while three others have been discharged.

Rowley said it appeared to be a “spontaneous attack and that the victims were selected at random.”

A 19-year-old man was arrested following the incident after being tasered by police.

Initial reports from London suggested gang violence could be to blame or the stabbings were a terror attack, but as the day drew on, mental health was thought to be emerging as a more significant factor.

Mr Rowley said: “Early indications suggest that mental health was a factor in this horrific attack. However we are keeping an open mind regarding the motive.”

But the motive was still unclear, and in the wake of a string of recent attacks in Europe, authorities said they were keeping an open mind and that “terrorism remains one line of inquiry being explored”.

As a precautionary measure, Londoners will wake to an increased presence on the streets of officers, including armed officers — an unusual sight in the city.

Mr Rowley urged the public to remain “calm, alert and vigilant.”

The knife victims were found injured in tourist hotspot Russell Square and were rushed to hospital. Three of them have reportedly been released from hospital, and another two remain. Authorities have not released information on their conditions.

Paramedics tried to save the woman, aged in her 60s, but she died at the scene.

A witness told the Express he saw a “body on the floor” and “lots of armed police”.

Another said the boots of the victim were visible sticking out from underneath a white sheet covering the body.

The suspect was initially taken to hospital after being Tasered, but has now been discharged and remains in custody at a south London police station, where he has been formally arrested on suspicion of murder.

The stabbings happened outside the Imperial Hotel. A forensic tent has been set up in the centre of the square, which is near the British Museum.

Police said they were called to the scene just after 11.30pm local time after reports of a man with a knife “injuring people”.

Several witnesses said they saw a man flee the scene on a motorbike. Met Police said they couldn’t rule out any other arrests.

The fact terrorism was mentioned so early by police, when they usually decline to speculate, could indicate early witness accounts included a description of what the attacker was saying as he carried out the attack.

“The normal posture is to calm the public and say we don’t have any information that it’s terrorism yet,” former FBI agent Bobby Chacon told CNN.

The Sun quoted a witness who went to see what was happening after they “heard a scream”.

“I saw a girl lying on the floor with blood coming from her back. Another girl had blood on her arm.”

The women were believed to be speaking Spanish, an onlooker told the Daily Mail.

The two women were with two men who were shouting “tio lo vi to do” in Spanish, meaning “I saw everything”.

“They all spoke Spanish so I guess they were a group. I only saw the victim and the girl with the arm stabbed from that group.”

A caller to BBC Radio London named Paul said he came across the scene, reported The Guardian.

“We came around the corner and there were armed police everywhere … a massive police presence … and a corpse lying in the street covered in a blanket.

“Apparently it was a gang stabbed a woman and she’s dead.

“We went into the bar across the street and the officers came to interview us … we didn’t actually see anything but some people in the bar saw something.”

Russell Square is a large garden square and a London Underground station on Bernard Street in Bloomsbury, in the London Borough of Camden.

The stabbings came just hours after the Met police chief and mayor announced more armed police will be on patrol across the capital in a bid to deter terror attacks.

Metropolitan Police commissioner Sir Bernard Hogan-Howe said a rise in armed officers will be placed across London, including at famous sites and locations.

According to the BBC, the number of armed officers are set to jump from 600 to 2800.

While the Police Federation of England and Wales warned it would take months of training before the officers could hit the streets, the Met said the majority would be on patrol by April next year.

London Mayor Sadiq Khan said police were seeking “to establish the full facts including motives” for the attack and urged the city’s residents to stay calm but alert.

“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.

Europe has been on edge for months following a string of recent jihadist-inspired attacks.

Last month a driver ploughed a truck through a crowd leaving a fireworks display in the French seaside resort of Nice, killing 84.

Less than two weeks later, two men stormed a church in northern France, slashing the priest’s throat at the altar. Islamic State jihadists claimed that both attacks were carried out by their “soldiers.” London was already on high alert following these attacks and others in Europe, including several in Germany.

Since August 2014, the terror threat level in Britain has been “severe” — the second highest level, meaning an attack is “highly likely”.

— with Debra Killalea