Spain said Ibon Gogeascoechea was the "most senior" member of Eta
One of the top leaders of the Basque separatist group Eta has been arrested in north-western France, the Spanish interior ministry has said.
Ibon Gogeascoechea was held with two other suspected Eta members in a French and Spanish operation in Normandy.
Madrid said the arrests had foiled a planned "commando" operation in Spain.
A militant group fighting for an independent Basque homeland, Eta has been blamed for more than 820 deaths during its 41-year campaign in Spain.
Eta called a short-lived truce in 2006, but broke it in December of that year.
The Spanish interior ministry said Ibon Gogeascoechea was the "most senior" member of Eta and its military chief.
The arrests took place close to the small Normandy village of Cahan.
The Spanish interior ministry said the three arrested men had raised suspicion after renting a rural home with false identities and using a car with fake number plates.
Interior Minister Alfredo Perez Rubalcaba said the operation was "very significant".
One of the other men was named as Beinat Aguinalde Ugartemendia, 26. Initially police named the third man, but later said his identity could not be confirmed.
Mr Rubalcaba said the pair "were part of a commando [unit] ready to enter Spain".
They had come to "say goodbye to the military chief, who gave them their final instructions as Eta has a habit of doing", Mr Rubalcaba said.
Ibon Gogeascoechea was born in 1965 and has been on the run since 1997 after members of the Eta group's Katu cell allegedly tried to kill King Juan Carlos when he attended the Guggenheim Museum in Bilbao.
The cell is also wanted for attacks on Burgos and Majorca.
French and Spanish authorities have maintained close cooperation to try to track down Eta members.
Four suspected members of Eta were arrested in Portugal and France in January.
Three weeks ago Portuguese police also seized half a tonne of explosives at a house they said was being used as a base by Eta.
Although there have been a number of arrests of leaders, Eta has remained active - the group killed three Spanish police officers using car bombs in 2009.
In December, Spain raised its terror alert level to two on a four-point scale.
Mr Rubalcaba said that despite recent arrests, Spain did "not rule out an attack by Eta".
Eta is considered a terrorist organisation by the European Union and the US.