Police have confirmed the man was tasered then taken into custody after the incident, which took place in London’s Russell Square at around 10.30pm local time. He remains in hospital under armed guard.

The man was seen “brandishing a knife” and attacking people in front of horrified onlookers.

Emergency workers tried to resuscitate the woman, aged in her 60s, but she was pronounced dead at the scene.

The other casualties — a woman and four men — are being treated for their injuries in hospital.

Police said they weren’t ruling out terrorism, but said early indications were that mental health was a “significant” factor.

A police spokesman said: “Police were called at 10.33pm to reports of a man seen in possession of a knife injuring people at Russell Square, WC1.

“Up to six people were found injured at the location.

“A female was treated at the scene but was pronounced dead a short time later.

“We await an update on the condition of the other persons injured and details of any other injuries.”

He added: “A man was arrested at 10.39pm and a Taser was discharged by one of the arresting officers.

‘Terrorism is one possibility being explored at this stage.”

A witness said he heard one of the victims screaming and speaking in Spanish. The man, who gave his name as Michael told The Daily Mail he saw a woman aged around 25 years old bleeding from her back and another woman of the same age with a wounded arm.

“I only saw the victim and the girl with the arm stabbed from that group,” he said. “I just saw blood just next to her. She was lying on the floor with a friend hugging her.”

A woman claimed on Twitter her brother, who was among a group of students at a theatre outing and witnessed the incident believed the victim may have been with a hen party.

She wrote: “Sadly my brother and his mates witnessed the stabbing in Russell Square.

“My brother said the woman was surrounded with her friend he suspected they were on a Hen night.”

Paul Hutchinson, a taxi driver who passed Russell Square just after the attack, said: “The pavement was all taped off by police, lots of armed police, lots of cars and the body was just lying on the floor. You could see the boots sticking out from under the cloth.”

A police forensics tent has been erected on Southampton Row, near the British Museum. The initial police cordon around the crime scene has been extended, the BBC reports.

The incident occurred in the same area where a suicide attacker blew up a bomb on the London Underground in the July 7 attacks 2005. Germaine Lindsay, 19, killed 26 people when he detonated his bomb on a Piccadilly line Tube as it travelled between King’s Cross Station and Russell Square.

The attack came as London’s Metropolitan Police announced armed police would be deployed on routine patrols on the UK’s high streets, sports grounds and shopping centres for the first time in mainland Britain.

On Sunday, Metropolitan Police chief Sir Bernard Hogan-Howe warned that a terror attack in the UK was a case of “when, not if”. Counter-terror police are also probing the attempted abduction of a serviceman at knifepoint from a road near RAF Marham in Norfolk on July 20.

Police marksmen with SIG semiautomatic carbines and Glock side-arms are to carry out foot patrols in London while vehicle stops by armed officers will increase.