An explosive device went off on a sea-front walkway in the town of Laredo, northern Spain, on Sunday, Spanish media reported.
Spain's Europa Press agency attributed the blast to Basque separatists ETA. There was no immediate comment from Spanish authorities, but the website of El Pais newspaper said there had been no injuries.
An anonymous caller said ETA had planted four bombs on the beaches of Laredo, Noja and Noja's golf course and they would explode between 6 a.m. and 9 a.m. EDT, Europa Press said, citing emergency services, the Reuters reported.