Police personnel work at a crime scene in Russell Square in central London. Picture: AFP

Police were called to the scene of the incident at 10.33pm local time after hearing reports of a knife-wielding man attacking people.

UK Metropolitan Police confirmed in a statement that up to six people were found, including an American woman who was treated at the scene but was pronounced dead a short time later.

The woman who died was in her 60s, police said, and those wounded included one woman and four men who had “various non-life threatening injuries”. Two remain in hospital but three others have been discharged.

Floral tributes rest against railings near the scene of a fatal stabbing in Russell Square, London. Picture: APSource:AP

The five injured are British, American, Israeli and Australian.

Metropolitan Police Assistant Commissioner Mark Rowley said the investigation “increasingly points to this tragic incident as having been triggered by mental-health issues.”

“So far we have found no evidence of radicalisation or anything that would suggest the man in our custody was motivated by terrorism,” Rowley said.

A 19-year-old man was arrested at 10.39pm local time after police used a Taser. He is in custody at a south London police station.

“A man has been arrested on suspicion of murder,” London police said.

A police officer guards the scene of a knife attack in London, England. Picture: GettySource:Getty Images

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

Detectives from the force’s murder and terrorism squads interviewed the suspect, his family and witnesses and searched several properties, and found no evidence of radicalisation, he said.

Police put more officers on London streets after the incident, which came just days after authorities had warned the public to be vigilant in light of attacks inspired by the Islamic State group elsewhere in Europe.

Police said they received “numerous” calls from members of the public at around 10:30pm (2130 GMT, 5:30pm. EDT) Wednesday about a man attacking people with a knife in the streets around Russell Square, a busy central area full of students and tourists.

Officers used a stun gun to subdue a 19-year-old suspect, who was arrested on suspicion of murder.

Armed police personnel patrol in London's Trafalgar Square. Picture: AFPSource:AFP

Helen Edwards, 33, who lives in the area, came out for a walk late Wednesday and found armed police near a subway station. In a city with vivid memories of the July 7, 2005, attacks on public transport — two of which struck near Russell Square — she immediately suspected that an attack had occurred.

“There is always that thing in the back of your mind,” she said. “You live with that threat of terrorism or other crimes in the back of your mind. It wasn’t a huge shock I guess.”

Ellie Cattle, 21, a student staying in a hotel near the square, said she heard police shouting: “’Put it down, put it down!’ “Then I heard what sounded like a gunshot, but it must have been the Taser,” she said. “After that they just stopped shouting. I didn’t hear any screams from anyone.”

Police officers and cleaners tend to the scene of a knife attack in Russell Square. Picture: GettySource:Getty Images

Xavery Richert, 22, a French tourist staying in youth hostel on the square, told AFP: “I was buying a beer when I heard a woman shouting who was being chased by a man.

“I thought it was a bag snatching ... she was not hurt. I came out for a cigarette, I went back, there were firefighters, police, and then I saw the body under a sheet. You could only see the feet sticking out.”

Russell Square resident Constantine Somerville said: “It’s such a safe area and very quiet especially at night — why would somebody commit an attack in such a quiet area?”

London Mayor Sadiq Khan urged the public to keep calm and remain vigilant, and encouraged people to be the first line of defence against any form of attack.

“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.

A caller to BBC Radio London who identified himself as Paul said he had been coming out of the nearby Bloomsbury cinema when he saw the incident.

“We came around the corner and there were armed police everywhere,” he said.

“(There was) 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.”

A body bag is removed from a crime scene in Russell Square in central London. Picture: AFPSource:AFP

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.

Central London resident Eric Patrick Andrew said he watched police and ambulances flood the crime scene.

“An army of police and ambulance plus what look like body bags,” he posted on Facebook.

A taxi driver told Sky News he saw a body on the pavement opposite the Imperial Hotel.

“The pavement was all taped off by police,” Paul Hutchinson said. “There was lots of armed police and other police and there was a body on the floor. You could see the boots of the body sticking out where the body was.”

Knives are the most common murder weapon in Britain, which has strict gun-control laws. There were 186 knife killings in the year to March 2015, according to government statistics — a third of all murders.

Police personnel work at a crime scene in Russell Square in central London. Picture: AFPSource:AFP

In the last three years London has seen two knife attacks by people inspired by radical Islam. In May 2013, two al-Qaeda-inspired London men killed off-duty soldier Lee Rigby in the street near his barracks. In January, mentally ill Muhiddin Mire tried to behead a London Underground passenger, shouting that he was doing it “for Syria.”

The Russell Square incident came within hours of an announcement by London police that they were putting more armed officers on the streets. The idea was to sustain public confidence following attacks by Islamic State-inspired groups in Europe.

Metropolitan Police commissioner Sir Bernard Hogan-Howe and Mayor of London Sadiq Khan yesterday announced the initiative of the increased presence of armed officers to provide “reassurance” amid warnings a terror attack in the capital was no longer an “if” but a “when”.

It is a big deal psychologically for Britons who are unaccustomed to seeing their beat police armed.

Police personnel remove bags of evidence from a crime scene in Russell Square in central London. Picture: AFPSource:AFP

The Met had announced the number of armed officers in their ranks would go from 600 to 2800.

The specially-trained officers are armed with SIG 516 semi-automatic weapons and Glock 9mm handguns with counter-terror specialists mounted on specially adapted BMW F800 motorcycles and SAS-style assault zodiac on the River Thames to get to scenes quickly.

A woman walks past a crime scene in Russell Square in central London. Picture: AFPSource:AFP

“Frankly we are trying to send a message to the bad guys,” Sir Bernard said.

For months Scotland Yard and Westminster politicians have warned the public of the high potential for attacks as seen in other parts of Western Europe notably France and Belgium.

The incident occurred in the same area as where one of the 7/7 bombs detonated in 2005.

Police in Britain do not carry guns for the most part — a principle that remains unchanged. Even with the additional armed officers, most of London’s 31,000 police officers will not be armed.

— Additional reporting by Charles Miranda