Explosions rattled seven Spanish cities Monday following telephone warnings from the armed Basque (search) separatist group ETA (search), in a resurgence of violence after months of keeping a low profile since the deadly Madrid train bombings by Muslim militants.

Officials said ETA chose a highly symbolic day for a fresh show of force — the anniversary of Spain's constitution, which established a system of regional autonomy the Basque group rejects.

The nearly simultaneous explosions in coffee shops, parks and other public places slightly injured 18 people. They stretched across Spain, with the apparent message that ETA can strike wherever it wants — even with security forces on high alert because of five blasts Friday claimed by ETA.

"Once again ETA has tried to scare us on a special day," Interior Minister Jose Antonio Alonso said. "But today has to continue to be the day of the constitution, not ETA's day."

The bombs went off in Valladolid, Leon and Santillana del Mar in the north, Avila and Ciudad Real in central Spain, Alicante in the east and Malaga in the south. They came about an hour after two calls from people claiming to represent the outlawed group to the Basque newspaper Gara.

Authorities had time to evacuate the places mentioned in the calls, although in at least two instances the explosions occurred in places not referred to by the callers.

That was the case in the town of Santillana del Mar, where 15 people were hurt by shards of glass or chunks of wood when a bomb destroyed a tourist information booth in a park. Three people were wounded in Ciudad Real when a bomb exploded while authorities were evacuating a coffee shop.

The blasts were apparently calculated to avoid loss of life. One Basque analyst said this showed the constraints ETA faces after the March 11 Madrid train bombings — which killed 191 people — and the ensuing nationwide revulsion over terrorism.

ETA is blamed for more than 800 deaths since the late 1960s, and was initially blamed for the March 11 attacks until the government acknowledged an Islamic link.

"ETA wants to sow fear. It wants to draw attention, but cannot afford to kill people. That's why they set off bombs the size of a bar of chocolate," said former ETA member Teo Uriarte, who now leads an association working for peace in the region.

Uriarte suggested the bombings might have been planned by young Basque militants acting in ETA's name without specific orders from the group's leaders.

Analysts in the Basque region say the group is deeply divided among newer, young members eager to keep fighting and older leaders more inclined to negotiate an end to the conflict.

More than 200 suspected ETA members or sympathizers have been arrested over the past two years in Spain and in southern France, and officials in Spain's previous, conservative government had said the group was on its last legs.

ETA staged a string of small explosions in Spanish resort towns over the summer, causing little damage and only a few minor injuries. It has not carried out a fatal attack since a May 2003 car bombing that killed two policemen in the northern town of Sanguesa.

Monday's attacks were the most spectacular blamed on ETA in almost a year. Police foiled a plot to blow up a train in Madrid last Christmas Eve, and in February they intercepted a Madrid-bound truck laden with bomb-making chemicals.

Alonso said Monday that despite years of police success, "ETA still retains operational capability."

Last month Batasuna (search), a banned party seen as ETA's political wing, proposed a new formula for peace talks with the government, raising hopes ETA might be prepared to end its armed struggle for independence. But days later, ETA issued a statement pledging to continue attacks against Spanish security forces.