Several thousand people turned out at short notice on Saturday afternoon in the French Basque capital Bayonne to protest against the arrests by French and Spanish police.
The five - ecology campaigner Jean-Noël Etcheverry, farmers' union activist Michel Berhocoirigoin, winegrower Michel Bergougnian and two freelance journalists, Béatrice Haran-Molle and Stéphane Etchegaray - were detained on Friday night, three of them in the house and two on the road leading to it.
An investigation has been opened into "association with terrorist criminals".
An arsenal found there contained 26 handguns, eight assault rifles, 12 machine pistols, thousands of bullets, detonators, several kilos of explosives and acetone powder, according to police sources.
The activists had intended to render the weapons unusable and hand them over to the authorities, according to the honorary president of France's Human Rights League, Michel Tubiana, whom sources at first wrongly named as one of the detainees.
The move was meant to revive peace talks between the Spanish and French governments and Eta, the activists said in a letter published on Friday evening.
Cross-party protests in Basque country
The arrests have sparked protests across the political spectrum in the French Basque country.
Some 20 MPs and local councillors declared their support for the activists on Saturday, declaring their "commitment to peace indisputable".
Despite Interior Minister Bruno Le Roux's boast that the raid was a "heavy blow to Eta", they included members of the ruling Socialist Party, as well as representatives of the right-wing Republicans, Centrists, Greens and Basque parties.
Socialist MPs Sylviane Alaux and Colette Capdieville and Senator Frédérique Espagnac signed the appeal and attended the demonstration, while Bayonne mayor Jean-René Etchegaray, of the centre-right UDI, addressed the protesters, accusing the government of "cynicism".
"The peacemakers who were arrested yesterday are treated like criminals while they were trying to build bridges," he said.