Thousands have protested in Madrid against a European court ruling which freed an ETA member from jail, opening up the way for the release of more jailed Basque separatists.
Ines del Rio was jailed in 1989 for her role in 23 assassinations and car bombings but was freed on Tuesday.
A day earlier the European Court of Human Rights said her continued detention was illegal because of Spanish court changes to parole and other sentencing rules made while she was serving her sentence.
The Strasbourg-based court ruled that the retrospective changes, which would have kept her in jail longer, violated her human rights.
Another 51 jailed ETA members have asked for sentence reviews since the ruling.
Victims of Basque separatist violence along with the ruling centre-right People's Party protested today over the Strasbourg court ruling.
ETA has been on ceasefire for two years after a campaign over four decades in which more than 800 people died.
"ETA assassinated my father when I was only 15 years old," said Maite Araluce, addressing crowds waving Spanish flags and chanting "justice".
"Now we have to see how these murderers walk free, smiling... I don't want revenge but I also don't want them to enjoy what they have stolen from us."
Del Rio, 55, originally received prison sentences totalling more than 3,000 years, though Spanish law capped the amount of time someone could spend in prison for murder at 30 years.
She had been scheduled for early release in 2008, but the courts extended her detention and that of dozens of other ETA members.
In 2006, Spanish courts ruled that time off for good behaviour should be subtracted from a prisoner's total sentence, not the 30-year limit.
That meant Del Rio would have had no chance of getting out in less than 30 years.
The European court ruling overturning this is uncomfortable for the People's Party, which has traditionally taken a hard line on ETA and has made Spanish unity central to its policies, in the face of demands from Basques and Catalans for independence.
But the government looked unlikely to defy the European court ruling.
Prime Minister Mariano Rajoy described today's demonstration as a "protest of solidarity with victims, not a protest against the court."
"The ruling is humiliating for the victims," Estaban Gonzalez Pons, a senior PP member of parliament, told reporters. "We hope Europe will see how much this ruling has hurt Spain."
Some at the protest said the PP should have ignored the ruling or fought it harder.
"The government may have to obey the ruling but what is inadmissible is that 24 hours later people are already getting out. We don't accept this immediacy. They should have tried to see what could be done," said Maria Luisa, a retired public health worker.
About 600 members of ETA are in prison in Spain.
Of those, 61 had had their early release cancelled under the change in the law which could now be overturned.