A bomb placed by the Basque separatist group Eta in the northern city of Logrono failed to explode last night after only the detonator ignited, Spanish news media reported.
The group telephoned a warning to a Basque newspaper at last night, and half an hour later, a small explosion was heard in a car park in the city, the capital of the Rioja region adjacent to the Basque country.
Police sources were reported as saying there could have been between 100 and 150 kilograms of explosives in the bomb, which was later deactivated.
Hours earlier, the separatist group vowed to keep up bomb attacks against the Spanish state, saying last year's unsuccessful peace talks showed there was no point in negotiating with the Socialist government.
In the Basque city of San Sebastian, police broke up a rally yesterday in support of Eta prisoners and arrested the leader of an amnesty movement and five others.