At least 200,000 people marched through Madrid on Saturday, demanding the government call off planned peace talks with the Basque separatist group Eta.
Some protesters say Eta must disarm before talks
The march was called by Spain's right-wing opposition and associations of victims of attacks by armed groups.
Participants carried banners reading: "Negotiations, not in my name".
Eta declared a permanent ceasefire on 22 March, and in May Spain's Socialist prime minister announced his intention to open direct talks with the group.
Some victims' associations say the government is dishonouring the memory of those killed by Eta over four decades - though others have distanced themselves from the demonstration organisers.
'Don't talk to terrorists'
Saturday's march began with a tour of landmarks scarred by Eta attacks, as well as sites where Islamic militants launched a string of bomb attacks on trains in 2004, killing 191 people.
Demonstrators carried pictures of some of Eta's victims
"You don't talk to terrorists unless terrorists say 'we want peace, we will give up violence and we will give the government the weapons', and they have not said that," one woman participant told the BBC.
Organisers said a million people attended the march, though police put the figure at 200,000.
It culminated with a rally in one of the city's main squares. Here marchers were addressed by Francisco Jose Alcaraz, whose brother and two nieces were killed in an Eta attack in 1987, and who now heads the Association of Victims of Terrorism (AVT).
The "interlocutors that the government considers valid are assassins who have killed nearly a thousand people," he said.
Another speaker was Mariano Rajoy, president of the main parliamentary opposition, the Popular Party (PP).
The party broke off co-operation with the government this week, after the Basque branch of the Socialist Party said it would pursue talks with Batasuna, Eta's banned political wing.
But not all victims' groups support the demands of the rally organisers.
"There are a lot of victims who don't want to enter this political game," Roberto Manrique, vice-president of the Catalan Association of Victims of Terrorist Organisations, according to news agency Reuters.
He was referring in part to the PP's continued suggestions that a possible role by Eta in the Madrid train bombings had not been ruled out - despite an investigation which said Islamic militants were responsible.
Speaking in north-eastern Spain on Saturday, the Spanish Prime Minister Jose Luis Rodriguez Zapatero said he was convinced most Spaniards supported opening talks with Eta.
"The great majority of Spaniards know what it means to submit to the pain and horrors we have experienced and at what point it is worth making peace," he said.