The arrest was made in France after months of observation, the BBC reports. It likely won't remain the final one: Spanish authorities will continue pressuring ETA, Interior Minister Alfredo Perez Rubalcaba vowed.
The arrest comes a day after ETA via Basque newspaper Gara declared a permanent cease-fire to demonstrate its "firm commitment toward a process to achieve a lasting resolution and toward an end to the armed confrontation."
Yet officials in Spain haven't been convinced by the promise.
Spanish Prime Minister Jose Luis Rodriguez Zapatero said ETA's announcement didn't go far enough. The group should instead renounce violence forever, he said in remarks broadcast by Spain's Antena 3 television.
"Those who see some element of hope in ETA's announcement need to know that the road ahead is still long, because the only thing that matters is the definitive end of the ETA terrorist group," Zapatero said, adding that there won't be any negotiations with ETA.
"We are not going to allow any trick," he said.
Madrid has reasons to be suspicious when it comes to peace negotiations: A first attempt at peace talks failed in 2006 when ETA militants broke a truce by killing two people with a car bomb at Madrid airport.
Moreover, Spain has little incentives to start negotiations with ETA now.
During the past years, French-Spanish police cooperation has thinned out the top ranks of the group, which has to rely on increasingly young and inexperienced personnel. Observers say the group is weaker than ever before.
The recent police successes and waning ETA support in the region have caused the political wing of the Basque separatist group to try to revive peaceful negotiations but Madrid said it won't talk to ETA unless it completely renounces violence.
Formed under the oppressive regime of Gen. Francisco Franco, Euskadi ta Askatasuna (Basque Homeland and Freedom), for four decades has fought for an independent state in northern Spain and southwest France and has been blamed for around 850 deaths. It is considered a terrorist organization by the European Union and the United States.
ETA's violent resistance actually dates to the 19th century when religiously conservative Basques disapproved of the too liberal style of governance in Madrid, which aimed for more centralization. The Basque region as early as the Middle Ages enjoyed special privileges and autonomy, although they weren't always fully honored by Madrid.
When the Franco government harshly cut privileges and tried to destroy Basque nationalism, ETA formed itself as a militant resistance group aimed at ending the oppression and installing a fully independent Marxist-Leninist Basque state.