Thousands of people took part in a demonstration organized by the Association of Victims of Terrorism on Sunday in Spain's capital to protest a ruling by the European Court of Human Rights, or ECHR, that resulted in the release from prison last week of a terrorist.

The ECHR ruling could potentially lead to the release of more than 50 members of the Basque terrorist group ETA from prison.

Two ETA terrorists have already been let out of prison in the past week.

Sunday's protest was organized in response to the release from prison last week of 55-year-old Ines del Rio Prado, an ETA terrorist who had been sentenced to 3,000 years behind bars in connection with the deaths of 23 people.

Del Rio walked out of a prison in the northwestern town of Teixeiro on Tuesday as Spain complied with the ECHR ruling.

She was freed hours after Spain's National Court ordered her release in accordance with the ECHR decision.

Del Rio spent more than 26 years in prison.

Spanish law establishes a maximum confinement period of 30 years, but when Del Rio became eligible for parole, authorities invoked the "Parot doctrine," under which eligibility for early release is calculated on the basis of each individual offense covered by the cumulative sentence.

Del Rio took the case to the ECHR, which found that the Parot doctrine - named for ETA convict Henri Parot - violates two articles of the European Convention on Human Rights.

About 200,000 people participated in the demonstration, according to organizers, while police did not provide any crowd estimates.

ETA, an acronym for the Basque language words for Homeland and Freedom, has killed nearly 900 people since taking up arms in 1968 to seek a Basque nation comprising parts of northern Spain and southern France. EFE