Swedish-Turkish writer Hamza Yalçın was detained in Spain over an Interpol warrant issued by Turkey on Aug 3.
The 59-year-old socialist writer was detained during the passport control at Barcelona airport while he was set to depart to London.
Yalçın was, on Aug 4, arrested by the Spanish court in charge, pending trial for extradition.
Speaking with his relatives on the phone, Yalçın said he was subjected to maltreatment during police custody.
A legal case was earlier opened against Yalçın for insulting President Recep Tayyip Erdoğan in an article that was published in Odak Dergisi magazine. Yalçın is also accused of making propaganda on behalf of a terrorist organization.