Mr. Irastorza’s arrest makes it “difficult for the terrorist group to reach any of its objectives,” the ministry said.

ETA has killed more than 800 people since the late 1960s as part of its campaign to establish an independent Basque homeland in northern Spain and southwestern France. The United States and the European Union consider it a terrorist organization.

The group has not killed anyone on Spanish soil since 2009, and its last victim was a French police officer, shot in 2010 during a botched car theft near Paris.

In October 2011, ETA declared a cease-fire but refused to surrender unconditionally and turn over all its weapons. Since then, the government in Madrid has continued to round up its members, in close cooperation with the French police and police forces in other countries. Security experts now consider the group unable to carry out significant operations.

Weapons have also been seized, including a cache that was found in a forest north of Paris last month, containing about 145 handguns. The Spanish government has since refused to comment on whether it believes the group had planned to hand over those weapons rather than use them to carry out an attack.