Officers seized 6.5 tonnes of gold and silver from the stores, where dealers buy valuables from members of the public, a police statement said.
They seized nearly €1.7 million ($2.3 million) in cash plus 52 cars and motorbikes in "a large-scale operation across Spain against money laundering and tax fraud in gold dealers' establishments".
Some 600 police and tax officials searched 88 sites including gold dealers' stores and homes, the statement said.
"The conglomerate, which used fake companies and fronts, also served to bring stolen jewellery into circulation and dodge taxes."
Authorities froze bank accounts and other assets worth €16 million in the raids.
Officials would give more details at a news conference on Thursday morning in the eastern city of Valencia, the statement added.