Three bombs exploded at popular seaside resorts in Cantabria, northern Spain on Sunday, after warning calls from the Basque separatist group ETA.
There were no casualties because the authorities evacuated the areas in response to the warning.
"We received a call at around 10.30 am from someone who said they represented ETA and told us ETA had planted 4 bombs on beaches at Laredo, Noja and Noja golf course," said an emergency services official.
The first bomb exploded at about 1015 GMT on a seafront promenade in Laredo, one of northern Spain's most popular holiday destinations, damaging the walkway, breaking windows and sending a 25 metre plume of smoke into the air, an official said.
Holidaymakers had been cleared from the beach 45 minutes earlier and took cover in local cafes and bars which drew down shutters to protect against the blast, witnesses told radio.
"There were no injuries because the area had been cleared and cordoned off," a national government spokesman said.
Many European schools have started, or are beginning to start their summer holidays.
The second bomb went off in dunes at Noja, about 19 miles (30 km) from Laredo, causing a loud blast but no damage, Spanish media reported.
Poor weather meant there were few people on the Noja beach but a police call to evacuate the area sent tourists running, blocking the road out of town to the city of Bilbao, media reported.
The third explosion was in Laredo, again causing no injuries, officials said.
ETA declared a unilateral ceasefire in March 2006, raising hopes for an end to the violence.
But an ETA bombing at
The group officially terminated the ceasefire in June last year, and since then the Spanish government has blamed it for four killings including civil guards across the French border.
ETA is blamed for over 800 deaths in its 40-year campaign for Basque independence in an area straddling northeastern