Armed Metropolitan Police counter terrorism officers take part in an exercise on the River Thames in London, Britain August 3, 2016. [Photo/Agencies]

Londoners woke up to a heavier armed police presence on Thursday as officers said they had arrested a 19-year-old man on suspicion of killing a woman and injuring five others in a knife-wielding attack in central London late Wednesday night, on the same day the authorities unveiled the capital's new, heavily armed anti-terror unit.

Police say early indications point to mental health as a factor, and described it as a random attack. Police identified the man as a Norwegian of Somali origin and said there was no evidence he had been radicalized.

The stabbing came on the same day the Met unveiled a significant addition to its anti-terror unit, in response to the high number of recent attacks elsewhere in Europe. Six hundred heavily armed marksmen — called counter terrorism specialist armed forces (CTSFOs) — have been deployed on London streets.

The new unit, which includes a fast response contingent operating on BMW F800 GS motorbikes, brings the total number of armed police officers in the capital to 2,800. On Sunday, Metropolitan Police Commissioner Sir Bernard Hogan-Howe said a terror attack in the UK is a matter of "when, not if" and said the terrorism threat level in Britain remained at "severe."

Europe is on edge after several attacks in France and Germany in recent months, including a truck-ramming that resulted in 84 deaths in Nice on Bastille Day.

In the UK more than 90 percent of police officers are unarmed, and unlike their counterparts in other parts of the world do not routinely carry pistols.

Each force has a small number of trained uniformed number of officers who carry pistols and semi-automatic weapons to guard key targets such as government offices and railway stations, but Wednesday marked the first time a dedicated heavily armed unit has been introduced.

On Thursday, officers were alerted at 10:33 p.m. to reports of a man attacking people with a knife at London's Russell Square. Six minutes later police used a Taser, or electric stun gun, to subdue the man and arrest him at the scene. The suspect remains in custody.

Paramedics attended to five victims, two women and three men. One woman – a US national in her 60s - died at the scene and Scotland Yard said the other victims have "various injuries." The victims are from the UK, US, Israel and Australia.

"Early indications suggest that mental health is a significant factor in this case and that is one major line of enquiry," Metropolitan Police Assistant Commissioner Mark Rowley said in a statement on Thursday morning. "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."

To contact the reporter: angus@mail.chinadailyuk.com