Four bombs exploded at popular seaside resorts in Cantabria northern Spain on Sunday, after warning calls from the Basque separatist group ETA and following a small explosion outside a Barclays bank near Bilbao.

One woman was hurt by a flying stone and another treated for shock.

The attacks, which marked the beginning of ETA's traditional summer bombing campaign, which targets Spanish holiday resorts as part of the group's four-decade struggle for an independent Basque state in northern Spain and southern France.

Spain's Socialist government says the guerrillas have been severely weakened by a string of arrests, and have called for their surrender after calling off peace talks two years ago.

Prime Minister Jose Luis Rodriguez Zapatero broke off a peace process with ETA in 2006 after the group killed two Ecuadorians when it detonated a car bomb at Madrid's Barajas international airport.

The government condemned the attacks on Sunday and reaffirmed its commitment to fight the group.

"The best way to get a long prison sentence in Spain at the moment is to join ETA," Spanish Interior Minister Alfredo Perez Rubalcaba told national radio on Sunday after the attacks.

The first Cantabria bomb exploded at about 11:15 a.m. British time 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.

"We received a call at around 10:30 a.m. from someone who said they represented ETA and told us ETA had planted four bombs," said an emergency services official. "There were no injuries because the area had been cleared and cordoned off."

WEAKENED, STILL FIGHTING

Many European schools have started, or are beginning to start their summer holidays and ETA's summer bombings are aimed at hurting tourism in Spain, the world's second most popular holiday destination after France.

The second bomb went off around 40 minutes later next to the lifeguard tower on the beach at Noja, about 19 miles from Laredo, causing a loud blast but no damage, media said.

A bomb disposal team was only 15 metres away when the small device exploded, 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 said.

The third explosion was next to a Red Cross post in Laredo, close to where the first device went off, officials said.

A woman was slightly injured when she was hit by a rock sent flying by the fourth explosion on a golf course at Noja.

A pregnant woman was treated for shock after the bomb exploded while she ate lunch nearby, a government official said.

Early on Sunday a small blast occurred outside a bank in the town of Getxo, damaging a cash dispenser and breaking windows.

ZAPATERO AND ETA

ETA (Euskadi ta Askatasuna, or Basque Country and Freedom), usually gives a warning before attacking civilian targets. It does not warn of attacks on police, politicians or officials.

The group is listed as a terrorist organisation by Spain, the United States and the European Union. It has killed more than 800 people since 1968, usually with car bombs or shootings.

Zapatero has ruled out further peace talks and says the guerrillas' only option is a unilateral surrender.

Spain's conservative Popular Party opposition has questioned Zapatero's will to force ETA to lay down arms and says he still toys with the idea of resuming the peace process.

"We'll always support the government in its fight against terrorism so long as its aim is - as I presume it is - to defeat the organization," said Popular Party leader Mariano Rajoy on Sunday.

The Cantabria blasts were the first attributed to ETA since May 14 when the separatists exploded a bomb without warning at the Civil Guard barracks in Legutiano, killing policeman Juan Manuel Pinuel-Villalon and injuring 4 others.

Later that month, police in southwest France arrested Francisco Javier Lopez Pena and three other ETA chiefs.