Etcheverry gave no other details about the purported arms transfer or the contents of the arsenal itself.
"We have the political and technical responsibility for ETA's disarmament, and it has been done," Txetx Etcheverry, a Basque environmentalist, said Friday.
"ETA has handed over its weapons to civil society. They are on French soil," he said.
Etcheverry gave no other details about the purported arms transfer or the contents of the arsenal itself, saying they were "confidential".
ETA, founded in 1959, says its pledge confirms it has brought the curtain down on its armed campaign for a Basque homeland, a territory that straddles the border between northwest Spain and southwest France.
In Madrid, the government on Saturday dismissed ETA's disarmament as a unilateral affair and bluntly warned that the group -- which it denounces as a terror organisation --- could expect "nothing" in return.
"It will not reap any political advantage or profit," said Inigo Mendez de Vigo, Spain's culture minister and its government spokesman.
"May it disarm, may it dissolve, may it ask forgiveness and help to clear up the crimes which have not been resolved," he said.
Analysts say ETA's arsenal is estimated at 130 handguns and two tonnes of explosives.
The group, blamed for the deaths of 829 people in a string of bombings and shootings on both sides of the Franco-Spanish border, says it gave up its armed campaign in 2011.
It has sought to negotiate its dissolution in exchange for amnesties or improved prison conditions for roughly 350 of its members being held in Spain and France.
Around a hundred of them are serving sentences of more than 10 years.
ETA also wants an amnesty for its members who are living under cover, which antiterrorism experts in France and Spain estimate at about 30 people.
Etcheverry said weapons "experts" would carry out "a series of checks" on Saturday, referring to a verification body that includes a former Interpol secretary general, Raymond Kendall, which is not recognised by either the French or the Spanish governments.
According to sources with knowledge of ETA's disarmament initiative, the weapons cache will be identified by map coordinates, enabling it to be found.
French police are on standby to take possession of the weapons, officials have told AFP.
An event is planned in the French Basque city of Bayonne on Saturday to mark "Disarmament Day", but Etcheverry warned that peace could be harder to find.
"Disarmament does not mean peace," he said. "The French and Spanish governments must help to resolve all the consequences of this conflict."
ETA is still considered a terrorist group by the European Union.
The head of the regional Basque government in Spain, Inigo Urkullu, last month called on the Spanish and French governments to "show ambitious vision and open direct lines of communication" with ETA.
But the Spanish government rebuffed the plea and instead demanded that the group "dissolve" and never reappear.
In a newly published letter, ETA said the process of disarmament had been "difficult", praising the autonomous Basque authorities while accusing Spain and France of being "stubborn".
Spanish Interior Minister Juan Ignacio Zoido had already made clear on Wednesday that there "would be no negotiations nor concessions" to ETA members in exchange for disarmament.