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Sadiq Kahn has urged Londoners to “keep calm and vigilant” as police investigate a possible terror-related knife rampage in Russell Square that left one woman dead.

Armed police have today stepped-up patrols in the capital after a man slashed passers-by at random, just after 10.30pm last night killing a woman in her 60s dead and injuring five others.

A 19-year-old was arrested six minutes later in Southampton row after being hit with a Taser.

Onlookers described dramatics scenes as people “screamed and ran away from a guy who was holding a knife” in the busy Bloomsbury square.

The Mayor of London said police are working to “extremely hard” to establish a motive for the attack which counter-terrorism officers are investigating.

He added that his “heart goes out to the victims of the incident and their loved ones.”

Mr Khan said: “I have spoken with the Commissioner, and the Assistant Commissioner, who have assured me that our police officers are working extremely hard to investigate exactly what has happened and to keep the rest of us safe.

"I urge all Londoners to remain calm and vigilant. 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.

"Our police officers are doing an incredibly difficult job on our behalf and they have my full support."

Assistant Commissioner Mark Rowley said early indications suggest mental health played a “significant factor” in the attack but terrorism remains “an active line of inquiry”.