A car bomb exploded early today near a police barracks in the northern city of Burgos, injuring dozens of people, the Interior Ministry said.
The blast was being blamed on Basque separatist group ETA.
The explosion caused major damage to the barracks and slightly wounded 46 people, mainly from flying glass, and 38 were taken to hospitals, said regional ministry representative Miguel Alejo.
The bomb left a crater which had filled with water from broken underground pipes.
"The car used to cause the explosion has been displaced some 70m (230ft) so that gives you an idea of the power of the blast," Alejo said.
Police and emergency services did not receive any warning that a bomb had been planted, but the explosion had the hallmarks of a terror attack by ETA, Alejo said.
Television images showed considerable damage to a 14-storey Civil Guard barracks building and many residential dwellings around it with windows and some walls blown in by the power of the explosion.
Burgos was once the capital of Spain and contains a historic city centre with important tourist attractions.
On June 19, a bomb attached to the underside of a car killed a Spanish police detective who investigates ETA. That attack was in Arrigorriaga, near the northern Basque port of Bilbao.
ETA has killed more than 825 people since it launched a campaign in the late 1960s for an independent homeland in Basque regions of northern Spain.
ETA declared what it called a permanent cease-fire in 2006, but reverted to violence within months after peace talks with the Spanish government went nowhere.