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 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 extremist groups jumped by 43 percent in 2009 compared with 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 Prime Minister 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 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 extremists around Europe to step up their actions before the trial.

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

But European anarchists are not always in step.

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

Three imprisoned members of Revolutionary Struggle claimed in a communique Tuesday 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.

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: 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 Dec. 14, thanks in part to anarchist infiltration.