Documents seized from leaders of El Salvador's most prominent right-wing group following an unsuccessful coup attempt last May describe a well-armed, well-funded organization, with international connections and a firm belief that the United States is seeking to install a leftist government there.

The papers were taken during the May arrest of retired major Robert D'Abuisson, a Salvadoran former intelligence officer, and other members of the Broad National Front he heads. The documents have been circulating in Salvadoran government and diplomatic circles. U.S. intelligence officials vouched for their authenticity.

They provide a detailed example of the methods, means and thinking of the right in El Salvador's escalating political violence, where both of the extremes are seeking to overthrow the U.S.-backed civilian-military junta that took control last October.

Among the papers was the 1980 diary of Capt. Alvaro Rafael Saravia, one of the men detained by D'Abuisson following the coup attempt. It logs the daily expenditures, arms purchases and the alleged composition of death squads with ties in Guatemala, Costa Rica and the United States.

Expenses totaling thousands of dollars are meticulously noted for haircuts, gasoline, car repairs, meals and the rental of safehouses. There are payments of $700 to bodyguards, $250 to the police and several $1,000 payments to at least two active officers in El Salvador's police.

One memo says to "contract 20 men at $280." Another authorizes $80,000 "for the Nicaraguan," believed by intelligence analysts to be a hired killer.

Shopping lists of arms are scattered throughout the diary. One calls for 10 shotguns with 500 cartridges, two .45-caliber pistols with 100 cartridges, four 9mm submachine guns with 400 cartridges, six vests, three radios, a camera, two cars and one .22-caliber rifle with sight and silencer.

Several references are made to an M10 silencer usually associated with the Miami underworld and such unusual items as a Starlight night-sight scope and .257-caliber ammunition.

According to Bob Barnes of the Bureau of Alcohol, Tobacco and Firearms, most of the material was non-military, but some items were "a dream list -- the best equipment in certain areas. There are enough weapons there to arm a small army to fight a small war," Barnes said.

According to the documents, D'Abuisson and businessman Alfredo Mena Lagos planned to take over the country last May 5, when they would -- with the backing of previously allied military units -- announce they had saved El Salvador from a left-wing coup only days earlier. A state of emergency would be declared and supposedly moderate members of the junta would be implicated and replaced.

D'Abuisson would then reveal that the defeated Marxist-Leninist coup was perpetrated by the Trilateral Commission, U.S. Ambassador to El Salvador Robert White, ousted members of the junta, leftist terrorists and, before his death, Roman Caholic archbishop Oscar A. Romero.

Romero was assassinated in March by unknown assailants alleged by the investigating judge to have been hired by D'Abuisson.

The Trilateral Commission, a select body of political and business leaders of the West's major industrialized countries, is often accused of trying to overthrow the left and install the right in power in the Third World. Its members have included President Carter and his national security advisor, Zbigniew Brzezinski.

The rightists allegedly planned to claim the commission's intention to "install a revolutionary junta . . . mixing Christian Democrats, socialists, communists, multinationals and guerrillas . . . [making] Central America a Cuban paradise."

Before the May coup attempt, as before a similar try last February, U.S. officials -- who back the current government and its intended reforms -- reportedly made it known that the United States would offer no support.

There is little doubt that U.S. influence with the Salvadoran military is waning, with rightists there seeing the United States as a longtime friend turned traitor.

D'Abuisson and his colleagues were freed 72 hours after their arrest for the attempted coup when military commanders overrode a decision of the junta to hold them for investigation. Two weeks ago, he was deported to Guatemala from the United States after entry in defiance of a State Department revocation of his U.S. visa.