Detectives were questioning five people today in connection with a series of high value robberies on gem dealers across the country.
Police believe the raids in London, the Midlands and Scotland, have netted the gang responsible millions of pounds in the past year.
Officers from the Metropolitan Police Service's London Crime Squad arrested four men and a woman, aged in their 30s and believed to be of South American origin, following an operation in Leicester City Centre just after 5.30pm last night.
The gang is believed to have targeted victims from overseas, who were in the UK for business and had travelled between London and the Midlands.
Victims were subjected to violence on many occasions and in some cases threatened with a knife, police said.
A large number of the stolen items were taken out of the country and have been traced to the United States.
The five are being held at a police station in the Midlands on suspicion of conspiracy to rob.
Detective Sergeant Paul Rodgers, of the MPS London Crime Squad, said: "They were an extremely close knit, organised group who had been operating as a surveillance team and spotters in order to select their victims and commit the offence.
"These arrests were as a result of a complex and challenging operation."