Two Australians are among five injured in a random knife rampage in central London that left one woman dead.

An American woman in her 60s was pronounced dead a short time after police and ambulance crews were called to the scene at 10:33pm London time on Wednesday.

A Norwegian man of Somali origin was arrested after the stabbing in the city’s Russell Square.

Sky News sources have identified him as Zakaria Bulhan, whose family emigrated when he was five years old, and who was living in south London. He had a history of mental problems.

The Department of Foreign Affairs and Trade said an Australian man and woman were injured in the attack.

Both have now been discharged from hospital. The woman has a back injury and the man has a chest wound.

Another Australian woman was at the scene, but was not injured.

The dead woman has been identified as Darlene Horton, 64, who was only hours away from flying home to the US.

Ms Horton was the wife of a university professor from Florida who has been teaching summer school in London.

The other injured were from Britain,the United States and Israel. No identities have yet been released.

Police said there was no reason to believe the man was radicalised and the victims were not in life-threatening conditions.

The 19-year-old suspect was Tasered by police around 10.40pm (London time) and restrained by using “minimal force”, according to police.

Police, who have the man under guard in hospital, told media that “mental health is a significant factor” and that he didn’t appear to have links to Islamic terror.

The Norway government has confirmed the man is a Norwegian citizen, but said he moved to the UK in 2002.

Speaking outside Scotland Yard in the early hours of Thursday morning, Assistant Commissioner Mark Rowley said it had been a “tragic incident”.

“Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry,” he said.

“But of course at this stage we should keep an open mind regarding motive and, consequently, terrorism as a motivation remains one line of enquiry for us to explore.”

A British woman calling herself Lauren told AAP her brother was among a group of University College London students walking back to campus from the nearby Gielgud Theatre when “they heard a scream”.

“They then saw the man run past them very quickly … the woman was out with a group of friends and they saw her lying in the street,” she said.

Her brother told her he believed some of those who were attacked were on a hen’s night.

It is not known if the man was operating alone, but The Daily Mail reported witnesses seeing three men fleeing nearby Queen Square shortly after the incident.

“One man fled on a motorcycle heading down a pedestrianised area,” an unnamed source told the paper. “He obviously looked in a hurry.”

Woman victim 'may have been part of Hen party group' – Witness to #russellsquare stabbing https://t.co/xBH7hJNFU9 pic.twitter.com/4UfeIbwT0t

— The Mirror (@DailyMirror) August 4, 2016

One witness told The Daily Mail that the suspect was seen to go “on the rampage” with the knife, while another witness said he had heard the victim scream before running over to help the group.

The members of the group, that was attacked and injured, were reported by witnesses to be speaking Spanish in the wake of the attack.

“We await an update on the condition of the other persons injured and details of any other injuries,” police said in a statement.

The US ambassador to London, Matthew Barzan, tweeted his condolences.

Heartbreaking news that a U.S. citizen was killed in #RussellSquare attack. My prayers are with all the victims and their loved ones.

— Ambassador Jane Hartley (@USAmbUK) August 4, 2016

The incident occurred in the same area where one of the 7/7 bombs were detonated in July 2005, and came only hours after police unveiled its new anti-terror unit and that it was beefing up police presence on London streets.

The London Ambulance Service (LAS) said it sent four ambulance crews, a responder in a car, an advanced paramedic and two officers to the scene.

A medical team of a doctor and paramedic from London’s Air Ambulance also attended in a car.

Paramedics gave the woman “extensive treatment” at the scene but she could not be saved.

-with agencies