A hero member of the public detained a man at the scene of an attempted armed smash-and-grab raid and held him until police arrived.
Five men allegedly tried to break the glass front of a watch store in Avery Row, Westminster, but failed to do so.
The balaclava-clad gang targeted the shop in broad daylight on Tuesday morning, less than 24 hours after another smash-and-grab incident in nearby Regent Street.
Metropolitan Police confirmed they were called just before 11.15am on Tuesday.
A spokesman for the force said: "At this stage, officers believe there were five suspects riding three mopeds, armed with a sledgehammer. All had helmets and balaclavas on.
"The men were seen to make an attempt to break the glass frontage of the shop but failed to do so."
The police spokesman said one man at the scene "fell off a moped and was detained by a member of the public for a short time until officers arrived at the location".
He added: "This man was arrested and taken to a central London police station where he currently remains in custody.
"The moped he was riding on was seized by police.
"There have been no reports of any injuries."
Anyone with information on the incident is asked to contact police on 101. Alternatively, contact Crimestoppers anonymously via 0800 555 111 or visit crimestoppers-uk.org.
Keep up to date with the latest news in west London via the free getwestlondon app.
You can set up your app to see all the latest news and events from your area, plus receive push notifications for breaking news.