Spaniards are being urged to turn out for demonstrations against Eta after a policeman was killed by a car bomb.
Calling for the rallies outside town halls, the Basque regional president said Eta was seeking to "upset Basque society, its political parties and institutions, confront us and divide us" .
The bomb, which went off in the northern Basque town of Hernani, was not claimed by any group but bore all the hallmarks of Eta.
Basque policeman Inaki Totorika Vega, 25, died shortly after arriving at hospital. A colleague suffered light injuries.
The officers were checking a suspicious vehicle which had been pushed into the middle of a street where gangs of youths had earlier gone on a vandalism spree.
The bomb, set off by remote control, exploded as the policemen went to move the vehicle.
The youth violence was thought to be related to the arrest earlier this week of several members of the Haika group, which authorities consider Eta's youth wing.
Speaking in San Sebastian, president Juan Jose Ibarretxe urged Basque politicians and civilians who supported Eta and the independence cause to rebel against the killings.