Hundreds of thousands of Spaniards rallied in Madrid on Saturday, demanding the guerrillas lay down their arms.
Protest organisers said around one million people attended what was the fourth demonstration against Eta in less than a year and a half, but police put the number closer to 200,000.
Draped in Spanish flags and carrying photos of their dead, families of Eta victims joined opposition politicians to attack the plan of Jose Luis Rodriguez Zapatero, the prime minister, to end 38 years of violence.
Survivors of Madrid's 2004 commuter train bombings joined the anti-government protest and called on Zapatero to probe deeper into the attacks that killed 191 people.
"We want to know the truth!" they chanted in the heat of Madrid's Plaza Colon, amid shouts of "Zapatero resign".
A two-year investigation into the bombings ruled out involvement of the armed Basque group but the opposition Popular Party (PP) and some victim groups say Eta may have played a part.
Madrid said it would talk to Eta
after it declared a ceasefire
"The government has the people's support to defeat Eta but it has no support to negotiate for political gains, or allow killers to achieve political gains that have cost so many lives," Mariano Rajoy, PP president, told reporters.
Speaking in northeast Spain on Saturday, Zapatero said he had public backing to push for peace negotiations.
"The great majority of citizens know the suffering we've lived through, and the majority know up to what point it's worth persevering with efforts to reach peace," he said.
The government declared plans to hold talks with Eta after the group announced a permanent ceasefire in March.
Eta is blamed for more than 800 deaths in its fight for an independent homeland in northern Spain and southwest France.