Spanish police have arrested 36 members of Segi, an outlawed youth group linked to Basque guerrillas ETA, Spanish media reported on Tuesday.
The operation was carried out across the Basque Country and Navarra, northern Spain, on the orders of a High Court judge and was still under way into the morning.
Leaders of Segi, which was declared a terrorist group by the Supreme Court in 2007, were among the arrested, El Pais reported on its website, citing security officials.
ETA was founded 50 years ago during the dictatorship of Francisco Franco and has killed more than 800 people in its campaign for an independent Basque Country.
Security forces believe it has been substantially weakened by hundreds of arrests in recent years in France and Spain but that it is still capable of fighting.
Spanish police were not immediately available to comment.
