In joint French-Spanish police operation, Josu Ternera - who was on run for 17 years - was arrested in French Alps.
Group gives French authorities information on location of arms caches as part of promise to give up all remaining arms by Saturday April 8th.
Activists say arrest of five people in French Basque Country was a political move to prevent the destruction of part of the organisation's arsenal.
Mikel Irastorza, aged 41, who has been on the run since 2008, was detained in a house in Ascain, a village near the Pyrénées mountains.
French officials said Iratxe Sorzabal had been wanted for 'at least 10 years' while David Pla had been sought by police since 2011.
The military leader of the Basque separatist group ETA has been arrested in France along with an alleged accomplice, Spanish officials say.