About 400 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 on Thursday.
It is believed 12 arrests have been made so far, 12 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 £3m to £4m – over the past 16 months.
The 20 properties raided included some in the Chelmsford, Braintree, Southend, Basildon, and Uttlesford districts and London, mainly in the east of the capital.