Spain's government on Saturday cautiously welcomed the support of ETA prisoners for a call for an end to the separatist group's armed struggle, but said it was not enough.

Imprisoned ETA members said Friday they would stick to the so-called Guernica agreement which last year called for an end to the banned Basque movement's decades-long violence.

Friday's "statement is unprecedented because for the first time it comes from ETA prisoners", government spokesman Jose Blanco told the press.

"This is an important and significant step but it is not what society and the government want because it is not definitive and it does not announce the end of ETA," he added.

Basque newspaper Gara said Friday that the prisoners had "announced their decision to join the Guernica accord".

Most of the 700 Basque prisoners "joined the dozens of political, union and social forces to bring about a scenario of peace and solutions" in the Basque country, it said.

The October 25 accord signed by several pro-independence left-wing parties, unions and Basque nationalist movements called on ETA to agree to "a permanent and unilateral ceasefire that can be verified by the international community, as an expression of its will to definitively give up its armed struggle".

The detainees' decision followed months of debate after members of pro-independence parties had shown them the Guernica appeal in prison.

El Pais newspaper, quoting anti-terror officials, said that about 10 percent of the prisoners took a hardline stance on the issue and continued to be critical of the appeal.

ETA has been weakened considerably following a string of arrests and has not carried out any attacks since August 2009.

Since January the group claims to have imposed a unilateral ceasefire but the government of Prime Minister Jose Luis Rodriguez Zapatero says ETA must renounce all violence definitively and unconditionally.

ETA is blamed for the deaths of 829 people in its four-decade campaign of bombings and shootings to force the creation of a Basque homeland in northern Spain and southwestern France.

Meanwhile, several thousand people rallied in Bilbao on Saturday to protest the recent sentencing of Basque secessionist leader Arnaldo Otegi to 10 years in prison for trying to rebuild ETA's banned political wing, Batasuna.

"No more trials. No more convictions. We need a democratic solution now," said the large placard hoisted by supporters of 53-year-old Otegi, who was sentenced on September 16 by Spain's National Court for belonging to a terrorist organisation.