The Salvadoran guerrilla command said today that U.S. reports of retaliation against the group that claimed responsibility for killing four marines here are "totally false" and amount to "impotent braggadocio" by the Reagan administration.

The command, in a communique broadcast over the guerrillas' Radio Venceremos, also reiterated its responsibility for assassinating the Marines. It vowed to "maintain our line of legitimate defense against those who have declared war on us." Two American civilians and seven persons of other nationalities were killed in the same attack June 19 at a sidewalk cafe here.

"It is also false that the Army has inflicted 21 casualties on us and struck a militarily important . . . blow against any of our organizations, much less using supposed, dream-world special operations with support from North American intelligence services," added the communique from the General Command of the Farabundo Marti National Liberation Front.

The front, the overall organization of five leftist guerrilla groups, was reacting to a statement Wednesday by Defense Secretary Caspar W. Weinberger that Salvadoran Army forces acting on U.S. intelligence had killed some of those who carried out the June 19 killings. Weinberger depicted the Salvadoran Army action as U.S. retaliation against terrorism following repeated administration threats to strike back at those who attack Americans abroad.

But subsequent Pentagon clarifications said Salvadoran troops attacked and inflicted 21 casualties on the guerrilla group that claimed responsibility for killing the marines, the Revolutionary Central American Workers Party, without asserting that the triggermen had been killed or captured as Weinberger initially did in a radio interview.

The Salvadoran Army, backing off still further, said here that Weinberger apparently had "misinterpreted" information from El Salvador, adding that Salvadoran troops attacked several workers party formations in recent months as part of regular operations. U.S. intelligence was employed along with Salvadoran intelligence, the Army spokesman explained.

The guerrilla command said today that "the poorly informed Mr. Weinberger" may have been referring to a June 26 clash in northern San Miguel province in which six combatants of the Revolutionary Central American Workers Party were killed or wounded.