Ranks of well-heeled, middle-aged people holding Spanish flags marched through one of Madrid's smartest districts on Saturday, holding placards with slogans like "Surrender in my name. No!"
Many came to protest against what they said was Eta's involvement in the 2004 Madrid train bombings that killed 191 people despite Western intelligence agencies and Spanish police pinning the blame on Islamic groups.
Walking behind leaders from Spain's conservative Popular Party (PP) opposition, including former prime minister Jose Maria Aznar, protesters held banners saying: "ETA, Socialists, Zapatero. Who's behind 11-M [March 11]?"
The theory that Eta was behind the attacks has been propagated by the biggest-selling right-wing newspaper El Mundo, and tacitly supported by many in the PP.
Despite denials from the Zapatero's Socialists, many on the right say the government is talking to Eta after it declared a permanent ceasefire in March.
Jose Francisco Alcaraz of the Association of Terrorist Victims told protesters: "We think the government is giving in and is benefiting the terrorists. Every day the terrorist group is becoming more emboldened," Jose Francisco Alcaraz of the Association of Terrorist Victims told protesters standing in the drizzle.
The peace process has hit a series of snags over the past months, with low-level violence in the Basque Country, the theft of hundreds of guns from a French arms factory and a high-profile Eta member going on hunger strike in prison.
Spain has said peace talks can only begin once it is certain that Eta is respecting its truce and has ended its four-decade long struggle.
Zapatero had told parliament that a peace process was going to be long, tough and difficult, but that it would be handled with determination, prudence and respect to the memory of victims.
However, political party Batasuna - banned for its links with Eta - has said peace talks will only advance once it is officially included in the government's negotiations on the future of the Basque region.