A man has been Tasered after he was seen pointing a knife at his own throat near Downing Street, Scotland Yard said.
The 42-year-old was challenged by police just before 9pm on Sunday but failed to respond so the electronic weapon was used.
The man had been shouting in the road and appeared to have a knife that he was pointing at his throat.
He suffered a minor head injury when he fell and was taken to hospital as a precaution.
He was later arrested on suspicion of possession of a knife and affray, and remains in custody.
Two police officers from the Diplomatic Protection Group fired the Taser, but they were in an armed response vehicle and were not among those guarding the Downing Street entrance.
It follows a separate incident last month when police outside Buckingham Palace used a Taser on another man who was also accused of brandishing a knife.
Talhat Rehman, 54, is alleged to have pressed a second knife with a 6in (15cm) blade to his chest while clutching a set of beads before a police officer restrained him with the Taser.