Demonstrators deplore government decision to grant Eta serial killer house arrest.
The group has killed more than 800 people in four decades of armed struggle for independence of the Basque country, a region that has considerable autonomy within Spain.
The national election is currently scheduled for March 2008.
Back to jail
In a related development, Alfredo Perez Rubalcaba, Spain’s interior minister, said on Wednesday that an Eta killer who was released from prison on March 1 after a hunger strike that left him gravely ill would be sent back to jail.
He would not be home to serve out the rest of his prison term under police surveillance, a possibility that had initially been raised.
Jose Ignacio de Juana Chaos has been declared healthy again and was removed on Wednesday from the hospital in the Basque city of San Sebastian where he had been recovering from a 114-day hunger strike.
He was taken to Aranjuez prison, just south of Madrid.
He has already served 18 years after being convicted in 25 deaths in a series of Eta attacks and now faces the last 14 months of a separate, three-year term for newspaper articles that were deemed to be terrorist threats.