The decision by the European Court of Human Rights has already led to the release of three ETA members imprisoned for lengthy periods for carrying out bombings and assassinations, ThinkSpain reported Monday.
The decision also could entitle the separatists to compensation from the Spanish government.
An estimated 200,000 people went into the streets of the capital Sunday to protest the ruling.
The ECHR ruling overturned Spain's so-called "Parot Doctrine," which gave one day of early release credit for every day of a prisoner's good behavior. While some ETA prisoners had been given sentences of thousands of years, in practice the maximum possible term was 30 years.
The court's ruling rejected the extremely long sentences, saying the early release credits must be applied to the maximum term, not the actual sentence.
Spain is not obligated to abide by the court's ruling although the county is a signatory to the European Convention on Human Rights.
Three ETA members have already been released since the ruling. Two of them, Ines del Rio Prada and Antonio Troitino, were convicted of several bombings in Madrid in 1986 in which 24 people died. Del Rio Prada had been sentenced to 3,800 years.
A third person, Juan Manuel Piriz Lopez, shot a former ETA member in the head in front of the man's wife and children.
If awarded any compensation, the released separatists are unlikely to see any of the money. It would be applied against the thousands of dollars the courts had previously ordered the ETA members to pay the families of their victims.
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