The man, identified as M.Z., was detained on Tuesday night in Tortosa town in northeast Spain on the basis of an Interpol warrant issued against him.
M.Z. had been staying at the home of an acquaintance in Tortosa for two or three days, the sources said, adding that there was no evidence he committed any crimes in Spain.
The man will remain in custody in Madrid pending a decision by Spain's National Court on whether to extradite him to Pakistan.