A ministry statement today said the eight allegedly urged attacks to be carried out in Spain and selected people to send to fight for Islamic groups in Iraq and Syria.
The arrests were carried out in the northeastern provinces of Barcelona and Girona and in the central areas of Avila and Ciudad Real.
No further details were immediately available. Spain has arrested dozens of suspected jihadist militants and recruiters in recent years.
On Tuesday, police said they had neutralized a jihadi cell with the arrests of two people who were part of a larger group allegedly planning to carry out attacks in Spain and other European countries.