London - Mayor of London Sadiq Khan on Thursday urged Londoners to remain "calm and vigilant" after a woman was killed and five people injured in a city centre knife attack.Police meanwhile probed possible terrorist links following the arrest of a 19-year-old man at the scene of the incident which took place late on Wednesday in Russell Square.Early indications, however, showed that mental health was a "significant factor" in the attack, according to Scotland Yard."The safety of all Londoners is my number one priority and my heart goes out to the victims of the incident in Russell Square and their loved ones," said Khan."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," he said."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."Paramedics treated a woman believed to be in her 60s but she was pronounced dead at the scene.Two women and three men were also injured but no details have been released about their condition.The arrested man is currently in police custody in a hospital.