A car bomb exploded outside a Civil Guard barracks in the northern Spanish city of Burgos on Wednesday, injuring 46 people, an emergency services spokesman said.
Part of the building's facade collapsed into the street when the bomb went off about 4.30am local time.
A spokeswoman for the Civil Guard, Spain's paramilitary police force, said the attack was probably carried out by Basque separatist rebels ETA.
Most of the injuries were cuts and bruises and no one was in serious condition, although 38 had been transferred to hospital in the historic cathedral town, according to the emergency services spokesman.
"All the windows are broken. It's very damaged so it's almost a miracle no one was hurt more seriously," he said.
ETA is held responsible for killing more than 800 people in the past 40 years in a campaign to carve out an independent Basque homeland in northern Spain and southern France. It typically uses shootings and car bombs, often parked outside police and army barracks.
The most recent killing blamed on the group was on 19 June, when a police inspector died in a booby-trapped car in the northern city of Bilbao.
Prime Minister Jose Luis Rodriguez Zapatero's Socialist government broke off peace talks with ETA after the rebels killed two people with a car bomb at Madrid airport in December 2006.