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The Fighting Communist Party, an offshoot of the Red Brigades, took responsibility for the killing in a telephone call to an Italian newspaper. ''We have struck the heart of the state,'' the caller said. The Senate President, Giovanni Spadolini, said in a statement today that there was a ''tragic thread of continuity'' linking the shooting of Mr. [Roberto Ruffilli] and previous attacks by guerrillas. ''This cruel and monstrous killing, which has hit both an intellectual and a politician, shows that terrorist squads are still trying to destabilize our institutions 10 years after the death of [Aldo Moro],'' Mr. Spadolini said.

ROME (Reuter) - A faction of the Red Brigades guerrilla group yesterday shot dead an Italian senator who was a close aide of new Christian Democrat Prime Minister Ciriaco de Mita, police said. Political commentators said [Roberto Ruffilli], 50, was a close aide of de Mita, leader of the dominant Christian Democrat party whose new government goes before parliament on Tuesday for a vote of confidence.

Gen. Roberto Jucci, commander of the Carabinieri, the paramilitary police, said at a news conference this morning that the arrest of 16 men and 5 women ''had almost completely broken up the Rome base of the organization.'' Arms Reported Seized In addition, he said, those arrested included two people, Fabio Ravalli and his wife, Maria Cappello, specifically linked to a killing that shocked Italy last spring because it seemed carefully aimed at the [Ciriaco De Mita] Government, which had just taken office after a long period of political turmoil. 'A Very Heavy Blow' The operation today struck ''a very heavy blow'' against the group, General Jucci said, asserting that ''the number of terrorists remaining in circulation are no longer very numerous.''