Those arrested included people who had posted online messages sympathizing with groups including the Basque terrorist group Eta, Al Qaeda and the now disbanded left-wing, Catalan nationalist group Terra Lliure, Spain's El Mundo newspaper reported.
The arrests in locations across the country are the second stage of Operation Spider — Spain's first dedicated operation against cyberterrorism — with the first phase in April netting 21 arrests.
The issue of abusive online political messages came to the fore in Spain in the wake of the murder of Isabel Carrasco, a politician with Spain's ruling Popular Party, in May.
Shortly after the killing, a 19-year-old Spanish man was arrested for sending out threatening tweets authorities believed were an attempt to justify her murder.
"Put them (the politicians) in a row, or side by side, and give me an AK-47 (machine gun) and you'll see how I get rid of these fascists," the young man allegedly tweeted.
"Damn that's how you do it. Let's kill all of them," the man from Spain's Valencia region wrote in another tweet.
Spain's Interior Minister Jorge Fernández Díaz then announced authorities would crack down on people posting offensive.