Armed robbers tried to smash their way into a watch shop in New Bond Street on Monday.
Scotland Yard said the attempted raid took place at Watches of Switzerland at around 11:05am.
Six men on three mopeds rode towards the store, rammed the door and hit it with a sledgehammer and an axe.
One of the gag sprayed people with a noxious liquid, believed to be ammonia, injuring a security guard. It's also believed that one of them was carrying a handgun.
Staff managed to operate security shutters and, although the gang left empty-handed, damage estimated at several thousand pounds was caused and the security guard remains in hospital. His injuries are potentially life-changing.
Police are appealing for witnesses.