Shortly before 1 p.m. a number of suspects, believed to be armed with axes and hammers, attempted to rob a jewellers in Victoria Street, central London.
A statement from the Metropolitan police said it is believed the suspects, who were on motorcycles, were unsuccessful.
One of the men was detained at the scene by a member of the public who wrestled him from a moped and held onto him until officers arrived.
Speaking to the Telegraph, the bystander, who didn't want to be identified, said: "It was like watching a scene from a Guy Ritchie movie.
"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 hoody.
He added: "Not putting his hoody 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?'
"I hope the police manage to get the rest of them."
The suspect has now been taken to a central London police station.
The others are believed to have made off towards Belgrave Square.
Police are asking any witnesses or anyone with any information to call police on 101 on Crimestoppers anonymously on 0800 555 111.