El Salvador's left-wing rebel alliance announced today virtual completion of the largest prisoner exchange with the Army in the nation's five-year-old civil war.

The Armed Forces already have released four imprisoned guerrilla leaders, and the rebels have freed "most" of the eight Army officers who are to be freed, rebel political representatives told a news conference here.

The agreement also provides for 60 disabled guerrilla veterans, who were living behind rebel lines, to receive safe passage out of El Salvador to six different countries. Nearly all of those 60 have left the country under the agreement, which was sealed a week ago, the rebel representatives said.

Salvadoran Armed Forces spokesmen were not available to comment on the announcement. But a Roman Catholic Church official and a diplomat, both in San Salvador, said in telephone interviews that they were aware that an exchange was in the works and that the details announced by the rebels sounded "reasonable."

The exchange came less than five months after the first prisoner exchange between the two sides. That swap, in May, resulted in the rebels' freeing of a captured deputy defense minister.

The highest ranking guerrilla leader to be released in the new deal was described as a "very senior" official of the Popular Liberation Forces, one of the five guerrilla armies in the Farabundo Marti National Liberation Front. The highest ranking Army officer to be freed was Maj. Napoleon Medina Garay, who had been in guerrilla hands for more than a year.

The Salvadoran church and the International Red Cross acted as go-betweens in the talks. Rebel leaders and the diplomat in San Salvador cautioned that the exchange did not signal a significant relaxation in tensions.

The news conference -- with four members of the of the Farabundo Marti National Liberation Front-Democratic Revolutionary Front -- was set up by the office here of an American public relations firm, Fenton Communications, and was billed as a special opportunity for U.S. reporters to meet with Salvadoran rebel representatives.

The journalists grilled the four at length over the guerrillas' attempts in recent months to force youths into their ranks. The rebel representatives stuck to their previous position that nobody was forced to join, although they acknowledged that some youths were held for a time to persuade them.

The rebels said they doubted that whatever good faith was established in the prisoner-exchange negotiations would lead to talks aimed at ending the war.

Ruben Zamora, one of the four rebel representatives, said that Salvadoran President Jose Napoleon Duarte had rebuffed several attempts by the rebels to set a date to begin a dialogue. "His answers have always been evasive," Zamora said. " President Reagan doesn't allow Duarte to negotiate."

Zamora said that the rebels had proposed dates in July and later September. The offers were conveyed by the Roman Catholic Church and by the Costa Rican government, Zamora said.

Duarte has said talks with the rebels would have to wait until he controls the nation's far right, which has raised violent opposition to talks in the past. Salvadoran officials have said dialogue would have to wait until the start of next year at the earliest.

The rebels asserted that the negotiations over the prisoner exchange had begun before the June 1 inauguration of Duarte and that the contacts were with the Armed Forces and not with the civilian government.

The rebel representatives charged that the four guerrilla leaders who were released in the prisoner exchange originally had been captured not by police or Army officials but by armed men in civilian clothes working closely with the National Police. They accused these alleged "death squads" of holding the four in private homes and torturing them before handing them over to the police.

[In a separate development, one of El Salvador's right-wing death squads, the Secret Anticommunist Army, broke months of silence with a threat to kill union leaders and striking workers, Reuter reported.]