Txeroki is suspected by police of killing two police officers in December 2007
The suspected military chief of the Basque separatist group, Eta, has been arrested in southern France.
Spanish Prime Minister Jose Luis Rodriguez Zapatero called the arrest of Mikel Garikoitz Aspiazu Rubina, alias "Txeroki", a "severe blow" to Eta.
Mr Garikoitz Aspiazu is suspected of the murder of two Spanish civil guard officers in France in 2007.
Eta is blamed for the deaths of more than 820 people in its 40-year campaign for an independent Basque nation.
The arrest of Mr Garikoitz Aspiazu, 35, which took place overnight in the Hautes-Pyrenees region of France, follows the detention of Eta's alleged political commander, Javier Lopez Pena, in a joint Spanish-French operation in the French city of Bordeaux in May.
However, the BBC's Steve Kingstone in Madrid says that, in the past, high-profile arrests have always been followed by fresh attacks and Eta is far from defeated.
KEY ETA ARRESTS
May 2008: Political leader Javier Lopez Pena held in Bordeaux
July 2007: Logistics chief Juan Cruz Maiza Artola detained in Rodez, southern France
October 2004: Long-time political leader Mikel Albizu arrested in Salies de Bearn, southern France
December 2002: Military leader Ibon Fernandez Iradi held in southern France
"Today Eta is weaker and Spanish democracy is stronger," Mr Zapatero said.
But he admitted Eta would still be a threat.
"It has not lost its ability to attack all citizens; it has not lost its ability to cause pain; but with this arrest it has suffered a severe blow."
The group resumed its campaign of violence in June 2007, following the failure of secret dialogue with the Spanish government. Correspondents say Mr Garikoitz Aspiazu was a key figure in the decision.
The moment when the suspected military chief of Eta was arrested
The Basque news agency, Vasco Press, said that Mr Garikoitz Aspiazu, whose nickname means Cherokee, had been detained along with another suspected Eta member in the town of Cauterets.
The French interior ministry did not provide any other details about the arrest, but said he was "suspected of being the perpetrator" of the murder of two Spanish civil guard officers in Capbreton on 1 December 2007.
"This arrest shows again the resolute commitment of the French police and gendarmerie in the fight against all forms of terrorism and illustrates once again the excellent co-operation between France and Spain in the fight against Basque terrorism," the French statement added.
The two Spanish civil guards were shot during a surveillance operation on suspected Eta members. Their deaths prompted thousands of Spaniards to denounce the separatist group at a march in the capital, Madrid.
French police arrested a man and a woman over the attack several days later - but said at the time they were looking for a third suspect.
Earlier this month, Spanish Interior Minister Alfredo Perez Rubalcaba said two recently arrested suspected Eta members had said Mr Garikoitz Aspiazu had told them he had participated directly in the shooting.
One of them had said he "heard Txeroki recognise that he was the assassin of the two policemen," he added.
Eta suffered a major blow in May with the arrest of Mr Lopez Pena, alias "Thierry", along with three other suspected members of the group.
Mr Lopez Pena is alleged to have ordered the December 2006 bombing of Madrid's airport, which ended the 14-month-old ceasefire with the government and killed two people.
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