The Bayonne ceremony took place in the presence of the local mayor, an Italian archbishop and an Irish priest, and civil society representatives.
The Chairman of the International Verification Commission, Ram Manikkalingam, said he hoped this important step would help consolidate peace in the Basque region.
Speaking on April 7, Spanish government spokesman Inigo Mendez de Vigo said Eta “will get nothing from a democratic state like Spain”.
Inigo Mendez de Vigo demanded that Eta not only disarm, but also clarify who carried out past its attacks.
In recent years, police in France and Spain have put Eta under severe pressure, arresting hundreds of militants, including leadership figures, and seizing many of its weapons.
Eta was set up more than 50 years ago in the era of Spanish dictator General Franco. Its first known killing was in 1968, when a secret police chief, Meliton Manzanas, was shot dead in the Basque city of San Sebastian.
In 2014 the International Verification Commission of inspectors said Eta had put some of its weapons out of action, but the Spanish government dismissed the move as “theatrical”.
Spain’s top criminal court has ordered the release of nine jailed militants from the ETA Basque separatist group.
The Audiencia Nacional’s decision follows a European court ruling which overturned Spain’s extension of Eta prisoners’ sentences.
Last month there was bitterness among relatives of ETA victims when Eta member Ines del Rio was released.
A violent 40-year campaign by ETA killed more than 800 people. The group declared a ceasefire in 2011.
Spain and some of its Western allies, including France and the UK, consider Eta a “terrorist” organization.
On October 21, the European Court of Human Rights upheld its 2012 ruling against Spain which opposed a legal practice known as the “Parot doctrine” – that allowed prison terms to be extended for people convicted of terrorist offences.
Spain’s top criminal court has ordered the release of nine jailed militants from the ETA Basque separatist group
The next day, Spain released Ines del Rio, who was serving a 30-year prison sentence for her role in bomb attacks in the 1980s.
The court decision on Friday was narrowly in favor of releasing nine prisoners and keeping two others behind bars. Nine judges were in favor, and eight were against.
The nine militants to be freed include Domingo Troitino, jailed for a 1987 attack on a supermarket in Barcelona which killed 21 people.
Domingo Troitino is the brother of Antonio Troitino, who was released from custody in London last month in line with the European court ruling.
The other eight are: Jokin Mirena Sancho, Isidro Maria Garalde, Jose Ignacio Urdiain, Jon Koldo Aginagalde, Joseba Koldobika, Elias Fernandez Castanares, Juan Francisco Gomez Lopez and Luis Maria Azkargorta.
A Spanish ruling in March 2006 meant that the most serious criminals in Spain could not qualify for early release.
But the European Court in Strasbourg said the ruling must not apply retroactively and so, it argued, dozens of Eta prisoners and other criminals convicted before that date should be released.
The Strasbourg ruling has been condemned by organizations in Spain representing the victims of Eta bombings and murders.