Heavily armed cops guarding Tube stations and tourist hotspots after Russell Square rampage

HEAVILY-armed police have been out in force at London's tourist hotspots today, with the capital on edge after a US woman was slaughtered in a possible terror attack last night.

Scotland Yard has sent out its new elite unit armed with semi-automatic rifles and handguns, with officers out at Horse Guards Parade, Trafalgar Square, and St Pancras International train station.

The heightened presence comes after a Norwegian-Somali suspect with "mental health problems" is thought to have knifed a woman in her 60s to death.

Five other people were treated for injuries after the alleged killer lunged at "anyone he could see".

The bloodbath near the British Museum comes after Met Commissioner Sir Bernard Hogan-Howe warned it is case of "when, not if" there will be terror attack on the UK.

But the force's assistant commissioner Mark Rowley has told people to keep an "open mind about the motive", and that "mental health was a significant factor".

Last night's tragic knife attack comes after Mr Hogan-Howe unveiled a plan to put more highly visible, heavily-armed police, at major landmarks to deter potential attackers.

The Met sent out 600 highly trained marksmen onto the streets as part of the new anti-terror drive yesterday - hours before the Russell Square attack at roughly 10.30pm last night.

The 19-year-old man was tasered by cops after he ran from the scene and was arrested on suspicion of murder.

The Met Police's new heavily armed force are decked out in grey Kevlar body armour, and carry weapons including sniper rifles, automatic assault rifles, submachine guns, handguns, and tasers.

They also have BMW F800GS motorbikes at their disposal to allow them to weave through traffic and attend to major incidents.

The new officers are masked to hider their identities, which helps in hostage situations, targeted attacks, and kidnap attempts.

The release of a new highly-armed unit on London's streets came against a backdrop of warnings from the Police Federation.

They announced it will take two years to deliver a promised increase of 1,500 highly-trained officers across the country.

Speaking outside Scotland Yard in the early hours of Thursday morning, Commander Mark Rowley said last night's killing was a "tragic incident".

He said: "Police were called just after 10.30pm last night to reports of a man, armed with a knife, assaulting people in Russell Square, Camden.

"Armed officers arrived at the scene within five minutes, they discharged the Taser whilst confronting and arresting the suspect.

"Six people have been injured, including the one woman who sadly died at the scene, and the others have received various injuries."

Mr Rowley added: "A 19-year-old man was arrested a shortly after 10.39pm and is currently in police custody at hospital.

"Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry.

"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 enquiry for us to explore."