Aitor Elizaran Aguilar is a joint political chief of the group, Spanish anti-terrorism experts say. He was arrested with another senior member, Oihana San Vicente, in the north-western seaside town of Carnac.
Eta is blamed for more than 820 deaths during its 41-year campaign for an independent Basque homeland. Spain and France have been working closely to crack down on the separatists. The pair were reported to be carrying handguns at the time of their arrest, officials said.
Mr Elizaran is regarded as the co-leader of the political apparatus of the group, and a member of its executive committee, Spanish anti-terrorism sources told the media. He was “elevated” to Eta’s political leadership when the group’s previous number one, Francisco Javier Lopez Pena – known as Thierry – was arrested in May 2008, sources told Spanish news agency Efe.
The arrests follow the detention of 10 senior members of Eta’s banned political wing, Batasuna, in Spain last week.