Five small bombs have exploded near highways in the Madrid area and another in northern Spain but none caused injuries.
The Madrid blasts, as Spain marked Constitution Day, followed telephone warnings from the Basque separatist group ETA. Interior Minister Jose Antonio Alonso called the blasts another example of senseless violence that only confirms the need to fight ETA with all the power of the democratic state.
The sixth explosion damaged a post office in the northern town of Navarra.
Meanwhile, authorities evacuated the airport in the northern city of Santander after receiving a bomb threat believed to be from ETA. Police searched the facility, but found no explosives.
ETA has been blamed for more than 800 deaths over nearly four decades as it pursued its campaign for creation of an independent Basque state in northern Spain and southwestern France.