Fourteen people suspected of stealing about £30m through violent robberies on country homes and cash machines have been arrested.
Some antiques from Ramsbury Manor have been recovered
The arrests, part of Operation Haul, were made in Worcestershire, Oxfordshire and Gloucestershire.
West Mercia, Thames Valley, Warwickshire, Wiltshire and Gloucestershire forces were involved.
Police said the robberies included some of the highest profile and highest value burglaries ever seen in England.
All of those arrested are being questioned over conspiracy to commit a total of 23 offences throughout the five force areas, including thefts from cash machines and burglaries at a number of country houses, shops and hotels.
The police investigation, codenamed Operation Haul, involved up to 100 officers and began in 2005 in response to the activities of an organised crime network operating throughout the south of England.
Det Supt Mark Warwick, of Thames Valley Police, said some of the offences involved "extreme violence".
The crimes police investigated included break-ins at Ramsbury Manor in Wiltshire, Woolley Park House in Berkshire, Stanton Harcourt Manor in Oxfordshire, and Rendcomb Manor in Cirencester and at Ombersley Court in Worcestershire, all between November and April.
He said £30m worth of crime had been investigated in 21 months and about a third of the stolen property had now been recovered, including a cash machine and half of the antiques stolen in the Ramsbury Manor raid.
Officers have also applied to recover the offenders' assets under the Proceeds of Crime Act.
"They would quite happily threaten people and use violence to achieve their aims," he said.
"Some people have suffered severe trauma and stress from the violence that was used or threatened against them.
"We had a common problem, we had a common issue we needed to deal with, we came together, and we actually worked together to solve this."
He said the operation would continue over the next few days and more arrests were expected.