ROME — A loosely linked movement of European anarchists who want to bring down state and financial institutions is becoming more violent and coordinated after decades out of the spotlight, and may be responding to social tensions spawned by the continent’s financial crisis, security experts say.

Italian police said Tuesday that letter bombs were sent to three embassies in Rome by Italian anarchists in solidarity with jailed Greek anarchists, who had asked their comrades to organize and coordinate a global "revolutionary war."

Identical package bombs exploded at the Swiss and Chilean embassies in Rome on Dec. 23, badly wounding the two people who opened them. A third bomb was safely defused at the Greek Embassy on Monday.

"We’re striking again, and we do so in response to the appeal launched by our Greek companions," the Italian group known as the Informal Anarchist Federation wrote in a claim of responsibility for the third bomb that was released by police here Tuesday.

Extreme left-wing and anarchist movements have existed for decades in Europe — waging deadly attacks across the continent in the 1960s and 1970s that trailed and became sporadic in recent decades. Officials, meanwhile, focused far more intensely on the threat of Islamist terrorism.

But the European Union’s police agency, Europol, reported this year that attacks by far-left and anarchist militant groups jumped by 43 percent in 2009 compared to the previous year, and more than doubled over 2007, with most of the incidents in Italy, Spain and Greece. Spain and Greece have been hit particularly hard by government cutbacks and unemployment resulting from a continentwide debt crisis. Italy has also been growing tense in recent months in response to austerity measures and a political duel between Premier Silvio Berlusconi and a former ally that for weeks threatened the government’s survival.

Last month, 14 letter bombs were mailed to embassies in Athens by a Greek group that urged stepped-up attacks by anarchists worldwide. Two of the devices exploded, causing no injuries.

"Anarchists-insurrectionists work to try to raise the level of clashes when there are problems" said Marco Boschi, a criminologist who teaches a course on terrorism at the University of Florence and has written about anarchists. "They exploit every occasion."

A Greek group calling itself the Conspiracy Nuclei of Fire claimed responsibility for sending the 14 mail bombs in Athens. Panagiotis Argyros, 22 and Gerasimos Tsakalos, 24, were arrested on Nov. 1 in connection with the mailings and were charged with terrorism-related offenses. At least a dozen suspected members of their group are due to go on trial Jan. 17 for other offenses.

The Conspiracy Nuclei of Fire called on militants around Europe to step up their actions before the trial.

"We will organize internationally and take aim at the enemy. We can’t wait to see the subversive elements flooding the streets and the guerrilla groups striking again and again," the group wrote.

But European anarchists are not always in step.

The solidarity boasted by the Italian anarchists who targeted the Rome embassies apparently irritated a Greek militant group, whose membership included Lambros Foundas, who was killed in a shootout with police in Athens earlier this year.

Three imprisoned members of Revolutionary Struggle claimed in a communique Tuesday evening that their group never carries out actions "that would result in the injury of someone, like a random embassy official." The Italian anarchists, in their claim of responsibility for the embassy bombs, said their cell was named after Foundas.

Italian officials have said the Swiss Embassy was targeted by the latest attack in Italy because intensified Swiss-Italian cooperation led to two well-known arrests of anarchists.

Swiss anarchist and environmentalist Marco Camenisch, a hero to many anarchists, was arrested by Italian police in 1991 and imprisoned over the 1989 murder of a Swiss border police officer. After serving nine years in an Italian maximum-security prison, he was extradited in 2002 to Switzerland, where he was later sentenced for the murder.

In April 2010, Swiss police with the help of Italian authorities arrested two men and a woman who idolized Camenisch and were members of an Italian eco-terrorist group. They were suspected of planning to bomb an IBM Corp. research facility near Zurich.

Chile, meanwhile, was targeted because a Chilean anarchist, Mauricio Morales, was killed when a bomb in a backpack he was carrying blew up in Santiago in 2009, Italian officials have said.

Alessandro Ceci of the Center of Superior Studies for the Fight Against Terrorism and Political Violence theorized the Italian anarchists may be trying partly to take advantage of the political climate in Italy: Premier Silvio Berlusconi has seen his parliamentary majority fall and just barely survived a no-confidence vote this month. In addition, protests against university budget cuts turned violent on Dec. 14, thanks in part to anarchist infiltration of student demonstrators.

"If I were an Italian (police) investigator, I’d be worried," Ceci said, comparing the highly charged atmosphere to that of the late 1960s, just before the Red Brigades leftist domestic terror group launched into action.

But political science professor Franco Pavoncello at Rome’s John Cabot University said he didn’t foresee a return to that era of leftist terror.

"If this were the anarchists’ goal, they would not be focusing on embassies," he said.

He noted that no one really knows how many people are behind the group that claimed responsibility.

"They are surely not of an international level, maybe European, but I would better describe them as the result of pathological behaviors often of an individual nature, and very domestic," he said.

Another Italian anarchist group, the Italian Anarchist Federation, which happens to use the same acronym as the one behind the letter-bomb campaign, discounted the possibility of highly coordinated and organized anarchist offensives in the future.

"Anarchism by its own nature is not a hierarchal organization, and all the participants enter in a confederation on the same level and act freely," said Donato Randini, who edits the group’s periodical.

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Gatopoulos reported from Athens. Martino Villosio, Frances D’Emilio and Alberto Mucci in Rome, Nicholas Paphitis in Athens and John Heilprin in Geneva contributed to this report.

(This version corrects anarchist group name from Association to Federation.)