The bomb exploded at 6 a.m. (0400 GMT) in the doorway of offices of Spain's governing Socialist party in La Pena district. A call in ETA's name was made to a regional traffic department a half-hour earlier saying where and when the blast would occur, the spokesman said.

Seven officers who were in the area helping to evacuate people and cordon off the zone were treated for light injuries, said the spokesman. He was speaking on condition of anonymity in keeping with police regulations.

A police patrol in the area had already spotted a suspicious package outside the Socialist party building around 5 a.m. (0300 GMT) and had begun evacuating residents from the area.

The explosion came a day after King Juan Carlos called for political unity in fighting terrorism in Spain. He was speaking after inaugurating Parliament following elections last month.

Thursday's blast caused considerable damage to the party's office and to nearby buildings.

"We had to evacuate old people, children, people who were in their houses sleeping at this time," Basque Interior Department chief Javier Balza told Cadena SER radio.

"The houses are badly damaged. In the end it's the people who suffer the madness of the terrorists," he added.

The attack was immediately condemned by the Socialist party.

"ETA must know that it will make no political gains with violence, because in a democratic country like Spain, nothing can be gained by violently disrupting democratic and lawful norms," the party said in a statement.

Dealing with ETA is one of the main challenges facing Socialist Prime Minister Jose Luis Rodriguez Zapatero in his second term in office.

His first term was marked by bitter disagreements with the leading conservative opposition Popular Party over how to combat the group. The Popular Party demanded tougher police action while Zapatero leaned toward peace talks.

ETA has killed more than 825 people since the late 1960s in its campaign for an independent Basque country in northern Spain and southwestern France.

The group declared a cease-fire in March 2006, but ended it after it failed to win concessions in negotiations with Zapatero's government. It has since killed two police officers and a former Socialist party town councilor in shootings and has carried out several bombings.