Police investigate whether group of Pakistani men helped finance “terrorist” activities.
Spanish radio reported a number of men were arrested over “falsifying identity documents in connection with Islamist terrorism”.
In a statement, police said the group allegedly stole passports in Spain and forwarded them onto Thailand, where they were altered before being sent back to crime gangs in Europe.
Police said they were investigating whether the group may also have supplied such documents to international “terror” groups.
Spanish police have carried out several raids against suspected Muslim extremists since the March 2004 bomb attacks on commuter trains in Madrid, which killed 191 people.
Those claiming responsibility for the attack said they had carried it out in the name of al-Qaeda.
Last month, six people were arrested in Barcelona on suspicion of tax fraud and possible links to financing “terrorist” activities, while a year earlier police arrested 15 people who were suspected of planning an attack in Barcelona.