A Basque separatist militant has been released from prison after Europe's top human rights court ordered Spain to free her.

The European Court of Human Rights said the detention of Ines del Rio was illegal, a ruling victims groups warned would let dozens of prisoners walk free.

The court also told Madrid to pay €30,000 compensation to Ms Del Rio, who was jailed in 1989 for her role in 23 assassinations and car bombings carried out by the Basque independence movement ETA.

She had served 26 years of a 30 year sentence and was scheduled for early release in 2008.

However, in 2006 the Spanish Supreme Court had introduced the Parot doctrine which retrospectively prevented prisoners like Ms del Rio getting early release.

Upholding an earlier ruling that had been appealed by Spain, judges at the ECHR overwhelmingly decided her rights had been violated by the retroactive changes to sentencing rules.

Groups representing victims of political violence called on the government not to comply with the sentence and not allow it to set a precedent.

Ms Del Rio was found guilty of murder, attempted murder and other crimes in a number of trials. Prosecutors said she transported explosives, put them in cars and detonated car-bombs.

The former prisoner was shielded from cameras by her supporters as she walked out of a prison in North-Western Spain.

The rights court ruled her continued detention violated two articles of the European Convention on Human Rights covering the right to no punishment without law and to liberty and security.

Spanish Justice Minister Alberto Ruiz-Gallardon said Spain's High Court would have to go through dozens of cases to see if anyone else would have to be released.

Mr Gallardon said the government would not have to pay Ms del Rio anything because she still owed compensation to her victims under the terms of her sentencing.

Around 600 ETA members are in prison in Spain.

The UK has conditionally released Antonio Troitino Arranz an ETA ex-prisoner arrested for extradition to Spain.