London Mayor Sadiq Khan and police called on residents to remain calm on Thursday after a stabbing attack in the centre of the city left a woman dead and five people injured.
Police said they arrested an unnamed 19-year-old man on suspicion of murder, transferring him to a London police station on Thursday morning after he was treated in hospital for unspecified injuries.
"I urge all Londoners to remain calm and vigilant ... We all have a vital role to play as eyes and ears for our police and security services," Khan said.
Armed officers detained the suspect after they discharged a taser at him around 10:30 pm (2130 GMT) on Wednesday, the Metropolitan Police said.
"Early indications suggest that mental health was a factor in this horrific attack. However, we are keeping an open mind regarding the motive," said Assistant Commissioner Mark Rowley, who heads the force's special operations.
One woman, believed to be in her 60s, was treated at the scene but pronounced dead soon after. Two women and three men sustained "various injuries," police said.
The attack came hours after the deployment began of 600 extra armed police officers on the streets of London to counter the threat of terrorism.
"As a precautionary measure, Londoners will wake up this morning to notice an increased presence on the streets of officers, including armed officers, today," Rowley said in a statement early Thursday.