Armed Basque separatist group ETA exploded five small bombs on motorways around Madrid on Saturday as Spain celebrated the 27th anniversary of its constitution with a ceremony in the parliament building.
The bombs went off at the side of three of Madrid's ring roads and one each on the motorway exits to Barcelona and La Coruna, said a source at Spain's anti-terrorist forces.
"Five bombs have exploded on roads around Madrid," the source told Reuters. "They are small and have caused no injuries."
The explosions, between 3 pm and 3:15 pm (1400 and 1415 GMT), were condemned by Spain's Interior Minister Jose Antonio Alonso.
"This is the latest example of senseless violence and only confirms that we must continue to fight ETA with all the forces of the law and the democratic state," Alonso said in a statement.
Basque police said motorway assistance association DYA had received a warning in the name of ETA that it had placed bombs around Madrid.
Earlier, Santander airport in northern Spain was reopened after being closed for several hours while police searched for a bomb after a warning in the name of ETA.
It threatened to launch a grenade attack on the airport and the building was evacuated and flights diverted.
The caller to newspaper Gara - ETA's usual channel for announcements - said the attack would happen between noon and 2 pm (1100 GMT and 1300 GMT) but the deadline passed without any explosion, officials said.