Authorities have arrested 52 people in a major crackdown on a suspected ring of antiquities looters from dozens of sites in southern Spain, the Spanish Civil Guard said Wednesday.

The raids took place in several provinces, including Seville, Madrid and Barcelona, where police searched at least 68 homes, said Civil Guard spokesman Jose Manuel Escudero. Some 300,000 pieces have been recovered, including gold coins, vessels, amphoras and other objects, he added.

He said investigators consider the operation to be the largest against archaeological looting worldwide.

Among those arrested were 30 suspected thieves, 13 middlemen and nine collectors, who appeared to have been often motivated by investment rather than artistic and historical interest, Escudero said. The Civil Guard said the collectors were interested in "selected" pieces of archaeology.

Those arrested were not immediately identified.

The investigations stem from documentation seized in 2005 in an operation in which 10 people were arrested for looting underwater archaeological sites off the coast of Cadiz, the Civil Guard said in a statement.

The Civil Guard said the suspects arrested Wednesday made daily trips to the sites that they had previously located with sophisticated metal detectors and excavation manuals, and did the looting at night. In most cases, they were helped by security guards at the sites, who gave them access, the Civil Guard said. It said some 31 archaeological sites had been looted all together.

The members of the ring kept the pieces of less importance for themselves, while the ones that they considered very valuable were sold for large sums of money to collectors in Madrid, Barcelona, Cadiz and Seville.

Some of the pieces were also sold in other European countries.