An expert commission monitoring a ceasefire in Eta's decades-long armed campaign released a video of black-masked members of the group presenting guns, bullets and explosives to monitors.
"The commission has verified that Eta has sealed and put beyond operational use a specified quantity of arms, munitions and explosives," the body's spokesman, Ram Manikkalingam, told reporters in the Spanish Basque city of Bilbao.
"The commission is confident that this step is significant and credible. We believe that it will lead to the putting beyond operational use of all Eta's arms, munitions and explosives," the Sri Lankan spokesman said.
Spain's conservative government shrugged off the move by Eta, which is classed as a terrorist group by the United States and European Union.
Interior Minister Jorgé Fernández Díaz reiterated the government's demand for Eta's "unconditional disbandment" and its "total and definitive defeat" by Spain's security forces.
"With all respect, we do not need these international verifiers," he told a news conference minutes before announcement. "The civil guard and police are enough for us."
But Eta's move sparked rare optimism in the Spanish Basque country.
"It is a small step, it is not sufficient, but it is a first and necessary step towards complete disarmament," the regional president Inigo Urkullu, a conservative nationalist, told a news conference after the announcement.
"This is a step on a journey with no return that should have on its horizon the complete and total disarmament of Eta," he added.
Eta is blamed for the deaths of 829 people in a four-decade campaign of shootings and bombings for an independent Basque homeland in northern Spain and southwestern France.