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
Rashan Charles, 20, died after being apprehended by Metropolitan Police officers after running into a shop in Dalston, east London.
The House of Commons Commission will consider how long the bongs should be halted following concerns raised by a number of MPs.
The chimes are being stopped to allow extensive renovation work to the Elizabeth Tower, the Great Clock and its bell.