At least 20 people have been taken in for questioning in the provinces of Guadalajara in Castilla-La Mancha, Ávila and Zamora in Castilla y León, Lleida in inland Catalunya, and Madrid.
They are said to have bought passports and national ID, or DNI cards from Dominican Republicans who had acquired Spanish citizenship, flown refugees to Istanbul and then put them in the hands of people-smugglers to transport them by boat to Greece.
Each of the refugees and migrants paid around €2,000.
The traffickers are said to be Spanish nationals originally from the Dominican Republic, and the ID cards and passports sent by post or in person before being given to migrants with similar physical features and ages.
Migrants then paid a given sum of money for the flight to Istanbul and paid the rest to the boat-owners to get to the Greek islands.
Once there, they stayed in flats owned by the gang until they were able to be transported to Spain.
Investigations have been ongoing since 2014, and members of the racket were also involved in dealing cannabis.
Huge hash plantations were found in the Madrid region town of Fuente del Saz del Jarama, fuelled by electricity siphoned off the mains.