Radio broadcasts said the operation was ordered by high court Judge Fernando Grande Marlaska and was still under way in the northern Basque Country and neighbouring Navarre.
Spanish police arrested at least 10 people in a series of dawn raids on Friday on a banned organisation which authorities say is the youth wing of the Basque separatist group ETA, state radio said.
Radio broadcasts said the operation was ordered by high court Judge Fernando Grande Marlaska and was still under way in the northern Basque Country and neighbouring Navarre.
High court and interior ministry officials were not immediately available for comment.
Those arrested are suspected of belonging to Segi, an organisation banned in January 2007 when the Supreme Court ruled it had links to terrorism. Segi is also listed as a terrorist organisation by the European Union.
In November 2009 police arrested 34 people accused of being Segi members, and the interior ministry said that action had dismantled the leadership of the organisation, which it says is ETA's youth wing.
Spanish security forces have weakened ETA by arresting many of its key members in recent years, often in cooperation with their French counterparts.
Those arrested include suspected military leader Garikoitz Aspiazu Rubina, alias "Txeroki" (Cherokee), arrested in November 2008, and Ibon Gogeascoechea, ETA's top leader who had been at large since 1997 until his arrest earlier this year.
Success in tackling ETA has strengthened interior minister Alfredo Perez Rubalcaba, who was promoted to deputy prime minister in a cabinet reshuffle on Wednesday, while retaining his interior portfolio.
Also in jail is Arnaldo Otegi, the leader of ETA's political wing, who has since called for a new truce.
The government, rejecting his call, demanded a halt to ETA''s armed campaign to carve out an independent Basque homeland, in which it has killed some 850 people over four decades.
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