Spain busts gang that 'funded Syrian al-Qaeda militia'
With Europol's collaboration, more than 350 agents took part in an operation in Madrid, further south in Toledo and the eastern province of Valencia.
Fourteen homes and other premises were searched by officers.
The source, who refused to be named, said the detainees were Spanish of Syrian origin.
"The organisation was led by a family clan that for years allegedly used a legal business structure to hide illicit operations with which they evaded tax authorities and laundered large amounts of money," the ministry said in a statement.
"The detainees allegedly took away money from each legal operation, substituting the real amount of the operation in the receipt with another lower value amount," it said.
A large part of the stash of cash amassed went to the opposition-held province of Idlib in Syria to "give support and financial backing to terrorist militia there."
Idlib, an opposition stronghold in northwestern Syria, has seen several power struggles between extreme Islamist and more moderate rebel groups.
Relatives of the suspects arrested in Spain are allegedly al-Qaeda members.
The money was sent via various methods including human couriers and the "hawala" system, an informal method of payment based on trust that is far more difficult to trace than bank transfers.
One of those detained had already been arrested in Syria in 2008 and sentenced for his participation in an attack and for belonging to rebel group Fatah al-Islam.
The operation comes just weeks after a Syrian man was detained on suspicion of funnelling money to European fighters linked to the Islamic State group so that they could return to Europe.