A dozen suspected members of the new leadership of Batasuna, the banned political wing of the Basque separatist group ETA, were arrested overnight, Spanish media reported.
The arrests, on the orders of investigating judge Baltasar Garzon, took place in the Basque Country and Navarre region in northern Spain, according to media reports that cited anti-terrorism sources.
Police were searching for documents linking the radical Basque groups, Askatasuna and Democracy 3,000,000 to Batasuna.
The two groups, which are seeking to run for Basque Country regional parliamentary elections in March, are suspected by the police of being fronts for Batasuna.
In national elections last year, two parties were prevented from running because they were suspected of being fronts for Batasuna, which was banned as a political party in 2003.
ETA has been blamed for the deaths of 825 people in a 40-year campaign for an independent Basque homeland on the Atlantic coast of northern Spain and southwestern France.