About 450 police officers have taken part in raids as part of an investigation into thefts of cars worth a total of up to £4m.
More than 20 properties were raided in Essex and in London at about 05:45 GMT.
Essex Police said 12 arrests have been made so far, 15 stolen vehicles have been found and £90,000 has been seized.
The joint operation was carried out by Essex Police and the Met Police, along with British Transport Police.
It follows an investigation into a gang suspected of stealing about 90 cars – worth a total of up to £4m – since November 2018.
Raids were carried out in the Chelmsford, Braintree, Southend, Basildon and Uttlesford districts and London, mainly in the east of the capital.
Acting Supt Lewis Basford said they had been “targeting a specific group that had been offending, taking high-end value vehicles” and had also seized hi-tech equipment used to steal vehicles without a key.
“We believe this gang are responsible for the theft of vehicles from across the whole of Essex, London and car parks linked to the British Transport Police, particularly high-end Range Rovers and sports cars”, he said.
“The offences that we believe the group are responsible for [involved] using technology using keyless entry… and equipment used to automatically start a vehicle without the key being present in the offender’s hand.
“We’ve also targeted individuals that are then responsible for the coding of new keys that then facilitate the offence of the cloning of the vehicle that then go on to unsuspecting victims who buy these vehicles.”
He said the raids were “just the start of the operation to tackle the theft of motor vehicles”,
“What we are showing [organised crime groups] is that there are no borders that we won’t cross with collaboration to take them out,” he said.