Madrid  - Spanish police have thwarted a suspected bid by several jailed members of the ETA terrorist group to break out of a prison in the southwest of the country, local media reported Sunday.

The outlawed Basque separatist group was planning to spring several of its members from a jail by helicopter, then deposit them in neighbouring Portugal to go into hiding.

Four suspects have been arrested in Huelva, in the Basque region, in connection with the plot.

Chief among the reported targets for the escape plan was Gorka Garcia Sertutxa, jailed for more than 100 years in 1995 for an attempted assassination plot against King Juan Carlos on the holiday island of Majorca.

It is thought that plot was discovered as part of the documents seized when the head of the ETA organisation, Franscisco Javier Lopez Pena, was arrested in southern France in May 2008.

On Saturday thousands of ETA supporters protested in the Basque coastal town of San Sebastian over the disappearance of ETA former prisoner Jon Anza, who was last seen in April.

ETA alleges France and Spain have played a role in his disappearance - a charge Madrid and Paris reject.  (dpa)