Officers in Spain said they were holding a 41-year-old Moroccan man and a 36-year-old Spanish woman after intelligence-led operations which remain ongoing.
A spokesman for the Interior Ministry said the man had returned to Spain from Syria where he was trained by fighters from al-Qaeda off-shoot al-Nusra but fell out with his mentors and joined ISIS.
According to investigators, the suspect was in charge of radicalising and recruiting young people in the Tolosaldea region of Spain.
He has been targeting disaffected teenagers, preferably of Moroccan origin, and forging close relationships with them.
Spanish authorities said the man encouraged his young followers to abandon their families and travel to Syria and carry out violent jihad for ISIS.
They said they were aware of at least one young Spanish-Moroccan man travelling to Syria with the suspect in 2013 where both received firearms training.
The recruited was killed in the Syrian-Iraqi conflict zone two years later.
His family said prior to his radicalisation he was a young man with no religious background and was not radical.
Anti-terror police are also questioning a woman with links to ISIS who was detained near Alicante as she attempted to travel to Islamic State territory on the Syria-Iraq border with her four children to join the bloodthirsty terror group.
Investigators said she carried out propaganda work through social networks, where she used very aggressive slogans, jihadist emblems and published extremely violent videos edited by herself showing a radical Islamist ideology in favour of the Palestinian cause.
She collaborated with the ISIS through a contact she had with a well-known recruiter in Syria.
She had gone through all the necessary procedures to move with her four children to the conflict zone to meet her ISIS fighter husband who travelled there in 2014 and held a senior position with the extremist group.