A 42-year-old man was Tasered after he was seen pointing a knife at his own throat near Downing Street in London, Scotland Yard said.
He was challenged by police just before 9pm yesterday 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.