Ibon Gogeascoechea is suspected of being the "most senior" member of Eta
Handcuffs found during a raid on Eta suspects on Sunday suggest the group may have been preparing a kidnapping, Spain's interior minister says.
Ibon Gogeascoechea was held with two other suspected Eta members in northern France on Sunday.
"Handcuffs are not the type of material Eta commandos normally have," Minister Alfredo Pezez Rubalcaba told Spain's radio Cadena Ser.
"One of the theories is that he was planning a kidnapping," he said.
There were no details on who may have been the target.
Mr Rubalcaba had warned in December last year that the Basque separatist group might be planning a big attack or a kidnapping.
The Spanish interior ministry said Ibon Gogeascoechea was the "most senior" member of Eta and its military chief.
He was the latest in a string of alleged leaders of the group to be arrested in the past two years.
He had been on the run since 1997, when he was allegedly part of a cell that tried to kill Spain's King Juan Carlos when he attended the Guggenheim Museum in Bilbao.
One of the other men arrested on Sunday in a joint French and Spanish police operation 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", and were "saying goodbye" to their boss.
He said members of active cells usually met with the group's leader before setting off.
The trio were being held in northern France pending extradition to Spain, local media reported.
Mr Rubalcaba said computer material, guns, some explosives and detonators were also found in the raid.