The 42-year-old 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.
Two police officers from the Diplomatic Protection Group fired the Taser.
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.
More top news
Rob Young has run more than a marathon a day for the last 12 months.
The hospital where the Duchess of Cambridge is due to give birth has had a superbug outbreak.
The London Marathon brings together thousands of people raising money for charity - not to mention some rather unusual costumes.