Spanish police have arrested 20 senior members of banned political party Batasuna as they attended an illegal meeting in the Basque town of Segura.
"These activities cannot be tolerated and if the police discover them, it seems correct that the judge should order an intervention," state prosecutor Candido Conde-Pumpido said.
Batasuna was banned five years ago because of its links to Eta.
Those detained also included members of Ekin, a group suspected of being behind violent street protests, and political party ANV, which has stood in local Basque elections but is suspected of Eta links.
Eta called off a ceasefire after peace talks with the Spanish government broke down. Its last major attack took place in December 2006, when a bomb at Madrid airport killed two people.
The organisation has killed more than 800 people since starting its fight for independence for Basque territories in northern Spain and southern France during the final stages of the Franco dictatorship, when Basque language and culture were suppressed.