Suspected activists of the Basque separatist group ETA on Friday shot dead a local politician two days before Spain's general elections, prompting a suspension of the electoral campaign.
Isaias Carrasco, 42, a former councillor for Prime Minister Jose Luis Rodriguez Zapatero's Socialist Party, was fired at from behind as he was leaving his home in the Basque town of Arrasate - also known as Mondragon - on the last campaign day, officials said.
Spain's democracy did not accept the violation of its basic principles, Zapatero said. "We do not admit it today and never will accept it," he vowed.
"We shall defend our institutions and freedoms," the premier said, vowing that the killers would be caught.
Mariano Rajoy, the conservative candidate for prime minister, called for unity against ETA and said the group needed to lose "all hope of reaching its political goals". ETA seeks an independent Basque state.
The Socialists, Rajoy's People's Party (PP) and other parties suspended their campaigns following the incident.
The killer was a tall man with a fake beard, according to police sources. Another ETA member was believed to have helped him flee by car.
Carrasco, who had been a local councillor during the previous legislature, did not have a bodyguard, unlike about 1,000 other Basque politicians and other personalities.
The attack occurred as Zapatero was finishing a speech at an electoral rally in the southern city of Malaga. The premier immediately left for Madrid.
The government had raised a terror alert in view of a possible attack by ETA, which has staged two bomb blasts during the campaign. Those blasts damaged a television mast and a Socialist party office, but caused no injuries.