A "significant seizure of arms, explosives and munitions" was made in the raid by French and Spanish police on a property in Louhossoa, near Bayonne, on Friday night, the Interior Ministry announced.
But the five people arrested were not known Eta members.
They were Jean-Noël Etcheverry, the head of an ecologist group, Michel Berhocoirigoin, a former head of the Basque Country chamber of agriculture, Michel Bergougnian, a member of a winemaker's cooperative, and two freelance journalists, Béatrice Haran-Molle and Stéphane Etchegaray.
Wrong name leaked
And police sources who leaked the names to news agencies made a mistake, at first claiming that the honorary president of France's Human Rights League, Michel Tubiana, was among those detained.
Tubiana, a lawyer, contacted the media to explain that he had planned to be there but could not go.
The group's intention was to destroy the arms and then hand them over to the authorities in an attempt to jumpstart the stalled peace process between Eta and the French and Spanish governments, he claimed.
In a joint letter, dated 25 October and published on the Mediabask website on Friday evening, he, Etcheverry and Berhocoirigoin said that the ordnance represented "about 15 percent of Eta's arsenal" and that they hoped that its destruction would "contribute to a democratic future without violence for the Basque Country".
Minister backs raid
The government did not agree.
Hailing the raid as "another heavy blow to Eta", Interior Minister Bruno Le Roux declared that "nobody has the right to proclaim themselves a destroyer of weapons", which could turn out to be evidence in anti-terror cases.
Madrid praised Spain's Guardia Civil for dealing a "blow to the aresenal of the Eta terrorist gang".
Eta, which was formed in 1959 when General Francisco Franco ruled Spain, renounced violence in 2011 but refused to dissolve and surrender its arms because it wanted negotiations about the 400 or so of its members in French and Spanish jails.
The governments, which hold Eta responsible for 829 deaths in bombings and shootings, have refused to negotiate.
Police operations against Eta continue - one of its last remaining leaders in hiding, Mikel Irastorza, was arrested in November and another member caught earlier this month, while an arms cache was found and seized in the forest at Compiègne near Paris in October.
Nationalists, rights groups protest
Basque nationalists organised a protest against the raid in Bayonne on Saturday afternoon and France's Human Rights League called the arrests a "blow to the peace process".
Tubiana denounced the raid as "obviously political" and a "PR operation", whose aim was to "turn a voluntary initiative into a seizure".
Activist group Attac claimed Le Roux had made himself look ridiculous, while Green campaigner and MEP José Bové called for the detainees' release and the president of the Corsica region, Gilles Simeoni, dubbed the raid an "ambush on peace in the Basque Country".
Local politicians also criticised it.
Jean Lasalle, an MP who is standing as an independent in next year's presidential election, slammed the "Machiavellian authoritarian drift of the French state", while Senator Frédérique Espagnac, a member of the ruling Socialist Party, declared "all her support" for "activists working for peace".