A bomb exploded on Monday outside the headquarters of the Basque Socialist Party in the town of Lazkao after a warning call from the armed separatist group ETA, police in Spain's Basque region said.

The blast, which comes ahead of regional elections on Sunday in the Basque region, caused extensive damage but no injuries, they said in a statement.

It took place at 3:00 am (0200 GMT), nearly two hours after Spain's DYA road assistance service received a telephone call from someone speaking in the name of ETA warning that a bomb would explode at that time, the statement said.

ETA guerrillas traditionally call the DYA to warn of its attacks.

Just minutes before the call was received at 1:10 am, police on a routine patrol had spotted a rucksack containing the bomb outside of the party headquarters in Lazkao, the statement said.

"The agents then cordoned off the area, evacuated a nearby bar and warned local residents with a megaphone," it said.

Polls published Sunday showed the moderate Basque Nationalist Party (PNV) could lose its nearly 30-year hold on power in the Basque region to the Socialists in the upcoming regional elections.

ETA has been blamed for the deaths of 825 people in a four-decade campaign for an independent Basque homeland straddling northern Spain and part of southwestern France.

It is listed as a terrorist organization by both the European Union and the United States.

The outfit called off a truce in June 2007, saying it had grown frustrated with the lack of concessions on the part of the central government of Socialist Prime Minister Jose Luis Rodriguez Zapatero in tentative peace talks.

Since then, ETA has carried out about 30 attacks and killed six people, including three policemen.