A woman was killed and five other people injured by a man with suspected mental health issues who went on a rampage with a knife in central London, an attack police said could be linked to terrorism.
Armed police were called at 10:33 p.m. (2133 GMT) after a man with a knife started to attack people in London's Russell Square, an elegant park near the site of a 2005 suicide bombing.
"Early indications suggest that mental health was a factor in this horrific attack," said London Metropolitan Police Assistant Commissioner Mark Rowley.
"We are keeping an open mind regarding the motive," said Rowley, who is Britain's most senior anti-terrorism officer. He earlier said one line of inquiry was that terrorism was a motive.
Police, who arrived within five minutes of being called, used a Taser electric shock gun while detaining the 19-year-old suspect. He was later formally arrested on suspicion of murder.
The investigation was being handled by homicide command with support from counter-terrorism officers, Rowley said.
The victim was treated at the scene but pronounced dead a short time later. The other injured - one woman and four men - were treated in hospital. Three were later discharged.
Police cordoned off the southern part of the square, which sits at the heart of London's university area and is close to landmarks such as the British Museum, for several hours as forensics officers examined the attack scene.
Later, workmen hosed blood off the pavement.
Britain says its terrorist attack threat level remains at "severe", the second-highest level, meaning a strike is "highly likely". Police had already promised to deploy more armed officers in the capital after a spate of deadly attacks in France, Germany and Belgium.
Attacks across Europe have heightened tensions between some communities, raised questions about the European Union's border policies and bolstered support for anti-EU far-right groups.
Police chiefs and security bosses have repeatedly warned that Islamic State fighters want to carry out attacks against Britain, a close ally of the United States.
