A bomb exploded outside a court house in Spain's northern Basque region on Saturday, causing considerable damage but no injuries, regional police said.

The detonation in the town of Tolosa was preceded by a telephone warning to a regional road rescue service by a man who said he spoke on behalf of Basque separatist group ETA.

The caller said a bomb would explode by the court house at 1:15 a.m. Saturday. Officers rushed to the scene where they found a rucksack and proceeded to cordon off the area, a police statement said. The rucksack exploded at the announced time and blew a large hole in the courthouse, breaking glass all around the building and damaging cars.

As an organization, ETA normally doesn't claim responsibility for its attacks until weeks after they occur.

ETA has conducted a violent campaign, including shootings and bombings, since 1968 in an effort to create an independent Basque state in northern Spain and southwestern France. It has killed more than 825 people.

ETA declared a cease-fire in March 2006, but reverted to violence in December of that year after peace talks with the government failed. The group — considered a terrorist organization by Spain, the European Union and the United States.

The Basque region is one of 17 semiautonomous regions in Spain and among the strongest economically. It has its own language and culture and is home to a strong nationalist movement.