In a letter addressed to the international community, Eta - which has waged a bloody campaign of bombings and shootings - said it was giving up all of its weapons.
"'Disarmament day' is tomorrow and we want to warn that still the process can be attacked by the enemies of peace," the group said in the letter in English and Spanish published by the BBC and dated April 7th.
In the note, Eta, which is still considered a terrorist group by the European Union, said it had abandoned "all its weaponry (arms and explosives) to Basque civil society representatives" and described itself as a "disarmed organization".
The group said Bayonne, a city in the French Basque region, will be the focal point of the disarmament process where it expected thousands of people to gather on Saturday.
Eta's announcement comes after the head of the regional Basque government, Inigo Urkullu, said last month that the separatist group planned to fully lay down its weapons by April 8th.
Urkullu at the time called on the Spanish and French governments to "show ambitious vision and open direct lines of communication" with Eta.
But Madrid, a strong opponent of Eta, rebuffed the plea and instead demanded the group "dissolve" and never reappear.
In its newly-published letter, Eta said the process of disarming has been "a hard and difficult task", praising the Basque authorities while accusing Spain and France of being "stubborn".
Earlier Thursday there had been doubts over the final handover of arms, with arrangements for the event remaining sketchy.
So-called "peacemakers" drawn from French civil society were supposed to hand over an inventory of the weapons on behalf of Eta on the sidelines of a "big popular gathering" in Bayonne.
But at source close to Eta said that negotiations were still under way with French authorities and that the actual handover of the arsenal - which experts say consists of 130 handguns and two tonnes of explosives - would take place outside Bayonne to ensure that it is "total and verifiable".
"This disarmament is essential to definitively turn the page on violence in the Basque Country, but must nevertheless be carried out in full respect of the rule of law", French Socialist party senator in the region Frederique Espagnac told AFP.
Spanish Interior Minister Juan Ignacio Zoido said on Wednesday there "would be no negotiations nor concessions" to Eta members in exchange for disarmament.