MADRID  Spanish and French authorities said on Wednesday they had arrested one of the highest-ranking members of ETA, dealing a blow to the Basque separatist group after a series of attacks in northern Spain.

Police detained Francisco Javier López Peña along with three other members of ETA in the southwestern French city of Bordeaux on Tuesday night, Alfredo Pérez Rubalcaba, the Spanish interior minister, said.

“Francisco Javier López Peña is, in all likelihood, at this moment the person with the greatest political and military weight in the terrorist organization ETA,” Mr. Rubalcaba said at a press conference in Dakar, Senegal. He cut short a tour of African countries Wednesday to return to Madrid following the arrests.

“This is not just another operation against ETA,” Mr. Rubalcaba said.

Mr. López surfaced as key representative of ETA in peace negotiations with the Spanish government which collapsed when the group detonated a car bomb at Madrid airport in December 2006, killing two people. According to reports published in the Spanish newspaper El Mundo, he had replaced Josu Ternera, one of ETA’s chief negotiators, at a meeting with Spanish officials in Switzerland earlier that month. He was closely identified by Spanish authorities with the decision to end a nine-month cease-fire.