The everyday hero was standing outside H Stain Jewellers when a gang - two on a moped and two on a motorcycle - pulled up and used axes and sledgehammers to smash windows and grab items on display.
As the robbers made their getaway, the member of the public quickly grabbed a man off his moped and wrestled him to the ground.
The bystander, who asked not to be identified, told the Daily Telegraph it was "like watching a scene from a Guy Ritchie movie".
He said: "These guys screamed up on the scooters and two of the pillion passengers jumped off and tried to smash their way into the displays with yellow axes. I think they grabbed a few watches.
"I waited for a second because I wasn't sure they had guns and when they tried to zoom off I grabbed one who was driving by the hoodie."
"Not putting his hoodie up was a fatal mistake as it gave me something to get hold of.
"He swore at me and I managed to yank him off his machine. I thought 'this is my bloody city - why should thugs like this get away with it?'"
The robber, 18, was later arrested on suspicion of robbery and taken into custody at a central London police station.
However the other three men have yet to be apprehended. And, owing to their motorcycle clothing, Scotland Yard are unable to give a detailed description.
Any witnesses or anyone that could assist police are asked call on 101 or call Crimestoppers anonymously on 0800 555 111.