A young gunman arrested this week turns out to be the head of the Basque separatist group ETA - the first member of an uncompromising and ultra-violent new generation to lead the organization, Spanish officials said Wednesday.

Police said they were preparing for retaliation from ETA over the arrest of 35-year-old Mikel de Garikoitz Aspiazu, whose alias is Txeroki, or Cherokee in Basque.

Spanish authorities previously said Aspiazu was in charge of ETA's commando units, which carry out attacks. But Interior Minister Alfredo Perez Rubalcaba said in an interview with Cadena Ser radio that Aspiazu was actually ETA's top leader, in charge of overall strategy. He did not say how or when authorities had discovered that he held that role.

Spanish officials have described Aspiazu as the most-wanted member of ETA and a potent symbol of a new generation of ETA members: young and extremely violent, disinclined to peace negotiations and with little to no ideological grounding - unlike founders and older members of a group that ostensibly espouses Marxism.

ETA has killed more than 800 people since the late 1960s in its battle to create an independent homeland in northern Spain and southwest France for Basques, a people with their own language and culture. The most radical Basque separatists say they suffer repression at the hands of the Spanish government

ETA declared what it called a permanent cease-fire in March 2006 and began peace talks with the government of Prime Minister Jose Luis Rodriguez Zapatero. But the negotiations went nowhere and ETA detonated a car bomb at Madrid's Barajas airport in December 2006, killing two people.

Aspiazu is believed to have been opposed to the truce from the outset, and he is described as having given the order to end it with the airport bombing.

Police arrested Aspiazu on Monday in the French town of Cauterets, near the border with Spain, along with Leire Lopez, a woman who is also a suspected ETA member. They were asleep in an apartment when Spanish and French police burst in. The pair were found in possession of two pistols and 100 grams (3.5 ounces) of hash.

He has been transferred to Paris where he is being held in police custody. French authorities have not said if he has been appointed a defense lawyer.

The Interior Ministry mug shot of Aspiazu shows a grim-faced man with stubble, an earring and close-cut hair, except for curly strands in the back. It is not clear when the photo was taken but it is probably several years old.

He rose to the top position after the arrest of Francisco Javier Lopez Pena, then believed to be ETA's boss, in May near Bordeaux, France, Perez Rubalcaba said.

"Txeroki ended up in charge of everything - the political apparatus, the so-called military apparatus. The one who ordered killings was Txeroki," he said.

Spanish and French officials say Aspiazu is a top suspect in the shooting of two Spanish civil guards in December in the French resort town of Capbreton.

Spanish security forces are on maximum alert because ETA will probably try to attack and show it is not in disarray, the minister said.

"We have to be prepared. One of these days ETA will try to show it is not weak, that even though Txeroki has been arrested, it can still act," Perez Rubalcaba said.

Perez Rubalcaba described Aspiazu as "a legend within the group."

His arrest is clearly a major coup but not a definitive one because ETA leaders have fallen before, only to see the group just designate another, the minister said. Spanish officials often describe ETA as a hydra.