VITORIA, Spain (AP) _ A bomb exploded inside a small shopping center in the Basque capital of Vitoria on Wednesday, gutting much of the building in the fourth attack in a week blamed on Basque separatists.

The bomb caused extensive damage to the building in a Vitoria suburb but exploded shortly after midnight Wednesday when nobody was around, the Basque Interior Ministry said.

The ministry said a man called a local newspaper and municipal police, identified himself as a member of the separatist group ETA and warned that a bomb would explode in the shopping area in five minutes.

The bomb, placed near an ATM, destroyed much of the headquarters of Caja Vital bank and blew out the roof of a food shop.

Enrique Villar, the central government's representative in Vitoria, said he knew of no specific motive for the attack.

``They don't need a motive,'' he said, referring to ETA. ``They have so much hate that they have to manifest it in this way.''

Street violence in the Basque region is common, with gangs of independence-supporting youth throwing gasoline bombs at government buildings. But the ministry said Wednesday's bomb was far more powerful than the homemade devices of the youth gangs.

The attack is the fourth blamed on ETA since a week ago when a car bomb exploded in a downtown Madrid shopping area, injuring nine people.

On Saturday, a city councilor in the southern city of Malaga was shot to death in the street in front of his daughter. The next day, a car bomb exploded outside police barracks in northern Spain, injuring one person.

The attacks are seen as an acceleration of the campaign of ETA, which has killed some 800 people in its 30-year-old war for independence for the Basque region of northern Spain.

Human rights groups and citizens' organization have called for a protest in downtown Madrid Wednesday evening to rally against the separatist attacks.