A car bomb exploded outside a police station in Spain's northern Rioja region after a warning call from the Basque separatist group Eta.
Yesterday's attack suggests that the terror group, which ended a ceasefire in 2006 and killed a former politician who worked for the ruling Socialist party two days before the general election on 9 March, is determined to remain high on Prime Minister Jose Luis Rodriguez Zapatero's agenda as he begins a second term in office.
The bomb went off in the city of Calahorra in an area that had been filled with people attending a Good Friday religious procession.
But the Civil Guard station targeted in the attack and surrounding buildings were evacuated before the blast, Calahorra's Mayor, Javier Pagola, told Spanish national television.
A police officer sustained a minor neck injury from the force of the blast, according to Civil Guard headquarters in Madrid.
Jose Javier Garcia, a Calahorra resident who lives less than 100 meters from the police station, told CNN+ television that he heard the blast from his home and saw damage in the street outside. "All the windows in the shops were shattered," Mr Garcia said.
A man saying that he represented Eta called highway police in the Basque region to warn of the bomb and the explosion occurred about half an hour later, El Mundo newspaper and the Europa news agency reported.
Eta has killed more than 800 people since the late 1960s in its battle to create an independent Basque homeland in northern Spain and south-western France.
Mr Zapatero began peace talks with Eta after it declared a unilateral truce in March 2006. But Eta grew frustrated with a lack of concessions and reverted to violence with a car bombing at a Madrid airport parking garage in December 2006 that killed two people sleeping in their vehicles. The group insisted then that the deaths were unintended and that the truce still stood. However, it declared the ceasefire formally over in June 2007.
Since ending the ceasefire, Eta has carried out more than a dozen, mostly minor, bombings.
Eta members shot dead two undercover Spanish police officers in southern France in December.