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
A man reportedly armed with several knives has been arrested in Whitehall on suspicion of planning a terror attack, police have said.
Metropolitan Police officer, Tom Harrison, has been crawling the course since the race began on Sunday morning.
The Old Bailey heard how Syed tried to get weapons online, including a bomb vest, and trawled the web to find a busy area launch an attack.