In Madrid, the government held a minute’s silence for the victims, while the prime minister, leader of the opposition and King Juan Carlos condemned the attack.
Jose Luis Rodriguez Zapatero was due to travel to the town later, before a trip to South America.
“I can assure you, gentlemen, that democracy will beat terror and that freedom will beat deadly fanaticism,” he said.
The attack came just a few days before a planned meeting between Zapatero and the head of the Basque regional government, Juan Jose Ibarretxe, who said the region did not deserve to have to witness another sad morning.
He added: “How much harm has ETA done – both at home and abroad – to the Basque people and the men and women who live here and, of course, the people killed by their violence, their cruel, cowardly and irrational violence?”
Polls show most Basques don’t want independence, although Ibarretxe is defying the Spanish government with plans to hold a referendum on whether to begin a debate on ties with Spain.