Mayor of London Sadiq Khan has called for the public to remain "calm and vigilant" as police investigate whether a knife attack that left one woman dead and five people injured was terror-related.
Khan said his "heart goes out to the victims of the incident in Russell Square and their loved ones" after the death of the woman, who has not been identified but is believed to have been in her 60s.
Two women and three men suffered "various injuries" in the incident in central London and are receiving treatment in hospital.
Armed police are being deployed on the streets of London on Thursday to reassure the public who were recently warned that it was a case of "when, not if" a terrorist attack took place in Britain.
Khan said police were doing an "incredibly difficult job" and the safety of Londoners was his "number one priority".
"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."
"A man was arrested at the scene. Police have spoken to him and are seeking to establish the full facts including motives for this attack.
"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."
A 19-year-old man, whose identity has not been released, is currently under armed guard in hospital after being arrested by police officers who used a Taser.
Police said they are keeping an open mind over the motive behind the attack, understood to have taken place near the Imperial Hotel.
They said terrorism was "one line of inquiry that we should explore", and that mental health could have been a significant factor in the attack.