A knife-wielding man attacked people at Russell Square, central London, killing one woman and injuring five. Police apprehended the suspect whose mental health raised concerns while possible terrorist motivations are also being investigated.

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The 19-year-old suspect, who had been held in custody at a London hospital, is now in a south London police station having been arrested on suspicion of murder.

Police were called in response to an ongoing attack at Russell Square, Bloomsbury, in the London Borough of Camden at 10:33pm local time on Wednesday night, according to the Metropolitan Police.

Upon arrival, officers discovered six people with wounds, with a woman in her 60s later dying from her injuries.

She was a US citizen, the London Assistant Police Commissioner says.

Spanish, Israeli, Australian and British citizens were injured in the attack, Sky News reports.

LIVE on #Periscope: Possible Terror Attack Russell Square https://t.co/Tu5NUixuJa — RT (@RT_com) August 4, 2016

“Up to six people were found injured at the location,” police said in a statement.

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

Two people injured in the knife attack remain in hospital, while three others have been discharged, according to London police, as quoted by Reuters.

Officers are currently on scene at #RussellSquare. Up to six people injured and one person has died — Metropolitan Police (@metpoliceuk) August 4, 2016

Police arrested the suspect after one of the officers took him down using a Taser.

A witness, Philippa Baglee, spoke to media at the scene.

"I just came out for a cigarette. There were people laying on the ground, and people standing round, and a guy with a motorbike helmet balanced on his head just walking up and down.

"And then the police car and the ambulance came, and that was it. I thought it'd been a motorbike accident, or somebody had been knocked over. I had no idea that anyone had been stabbed."

Scotland Yard’s counter terrorism unit known as SO15 joined the investigation after details of the incident emerged, the Guardian reports.

The London Ambulance Service was dispatched to the site. Additional police units were deployed to the area to provide reassurance.

Police cordon around Russell Square eyewitnesses report armed police pic.twitter.com/TfLFXdJpRx — Raheem Kassam (@RaheemKassam) August 4, 2016

“Terrorism is one possibility being explored at this stage,” the official statement added.

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

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

I think there's been an incident outside the Imperial Hotel on Russell Square in London. :( #Londonpic.twitter.com/t0plUsWM1p — Stuart Kenny (@StuartKenny33) August 3, 2016

One woman and four men received “various injuries,” according to ITV news. The deceased female was in her 60s, police added.

Russell Square London: forensic teams investigating possible terror attack pic.twitter.com/LNvUOXzKCD — Polly Boiko (@Polly_Boiko) August 4, 2016

London mayor Sadiq Khan is urging the public to remain calm.

"The safety of all Londoners is my number one priority," Khan said in a statement.

"Please report anything suspicious to the police. 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."

My statement on the incident in Russell Sq. I urge Londoners to remain calm & vigilant while we establish full facts https://t.co/38re2Qr0y6 — Sadiq Khan (@SadiqKhan) August 4, 2016

The roads and pathways around Russell Square have now reopened following the incident.

Police cordons had been in place as investigations were carried out by forensic officers.

A4200 Russell Sq reopened SB between Bloomsbury Pl & Tavistock Sq following a police incident, and traffic is flowing well for time of day. — TfL Traffic News (@TfLTrafficNews) August 4, 2016

Earlier, Metropolitan Police chief Sir Bernard Hogan-Howe announced that additional armed police would be deployed to patrol London's streets following a wave of terror attacks across Europe.

“I want the public to know that we have substantial resources and a range of tactics that we can call on to protect them," Hogan-Howe said, adding that attack in the UK was a case of "when, not if."

READ MORE: Anti-terror cops on permanent alert for France-style attacks in UK

Announcing the start of Operation Hercules, Hogan-Howe and Khan disclosed that they would increase the number of firearm officers on patrol by 600 to 2,800.

"We will see more armed officers on our streets, but there is no reason to be alarmed,” Khan said. “All of our police officers are playing their part and working closely with all of our communities to prevent the possibility of an attack.”