Eta killed more than 800 people and wounded thousands in more than 40 years of violence aimed at creating a Basque state out of territory in south-west France and northern Spain.
The group declared a ceasefire in 2011 but did not disarm.
Spain refuses to negotiate with Eta.
Eta says it is handing over all its weapons and explosives.
The pledge to disarm came in a letter obtained by the BBC, confirming earlier reports about Eta disarmament plans.
The letter, dated 7 April, says that "after giving up all its weaponry (arms and explosives) to Basque civil society representatives, [Eta] now is a disarmed organisation.
"We want to warn that still the process can be attacked by the enemies of peace," the letter says. "The only real guarantee to succeed are the thousands of people gathering tomorrow [Saturday] in Bayonne [south-western France] supporting the disarmament."
Speaking on Friday, Spanish government spokesman Inigo Mendez de Vigo said Eta "will get nothing from a democratic state like Spain".
Read the text of the Eta letter in English here and in Spanish here.
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.
A key figure in Eta's move away from violence is believed to be the leader of the pro-independence Basque left, Arnaldo Otegi, who has served several jail terms for his links to the organisation. However, some Eta prisoners criticised him last year for compromising too much on their behalf.
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 international inspectors said Eta had put some of its weapons out of action, but the Spanish government dismissed the move as "theatrical".
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