Those arrested were allegedly planning terrorist acts similar to ones carried out in other countries and were recruiting people to fight in Syria and Iraq, the interior ministry said in a statement.
The suspects were operating in Barcelona, Girona, Ciudad Real and Ávila, according to the interior ministry. The ministry said that the operation remains open.
Spain has been conducting raids on a series of suspected extreme Islamist cells in recent months, most of them in Ceuta and Melilla, two Spanish-governed coastal cities fenced off from northern Morocco.
Most of those arrested have been accused of recruiting militants for Islamic State fighters rather than actually planning attacks themselves.Two suspected jihadists were remanded by a judge on Thursday, ordered to be detained pending a trial on charges of belonging to a terrorist group and illegal arms possession.
Police arrested the two Spaniards "of Moroccan origin" on Tuesday in Ceuta, accusing them of belonging to a group "prepared" to launch possible attacks in Europe, the government said.
One of them had been on the run since May 2014, after being convicted by a court in southern Spain of assault and arms possession, a judicial source told AFP.
The government said the two are suspected of following instructions issued online by the leader of the violent extremist group calling itself Islamic State, which controls parts of Syria and Iraq.
The two suspects were linked to four other arrests in Ceuta on January 24th.
Spanish authorities say about 100 people from Spain are suspected of having joined jihadist fighters in Iraq and Syria, and fear they may return to launch attacks.
Hundreds more such radicals from France, Britain and Germany are also thought to have travelled to those countries to fight.