More than 400 officers took part in the swoop - codenamed Operation Symphony - which was one of the largest ever conducted by Thames Valley Police and covered Berkshire, Oxfordshire, Hertfordshire and Middlesex.
Scrap metal merchants targeted in Berkshire were A4 Metal Recycling Limited at Beenham Industrial Park, and the A1 Group at Silver Birches, Highland Avenue, Wokingham.
At Beenham officers arrested a 46-year-old woman, and five men aged between 20 and 48, while in Wokingham they held another five men between the ages of 22 and 41. They were all questioned on suspicion of receiving stolen goods or concealing criminal property, before being released on bail until dates in November.
Police are still carrying out searches at a number of the raided addresses and are expected to continue for several days.
Property seized so far from includes more than £400,000 cash, Network Rail track, nine tons of copper, 15 tons of Scottish Southern electricity cable, cast iron drain covers, a war memorial plaque, four JCB digger buckets, BT cable, a large quantity of what appears to be crushed antique household and church silver, unlicensed firearms and prohibited weapons and class A drugs.
Detective Inspector Rodger Cartwright, investigating officer in the operation, said: "Yesterday's operation proved very successful."
He added: "Metal theft is one of the fastest growing crime types nationally. It is not a victimless crime and can threaten public and community safety, as well as the national infrastructure."