The alleged ringleaders of a right-wing plot to overthrow the U.S.-backed junta in El Salvador were released from jail there last night in a surprise move that drastically increased tensions among disputing government and military factions.

U.S. officials had hoped that retired Salvadoran intelligence officer Robert d' Aubuisson, the head of the right-wing Broad National Front and an alleged camp conspirator arrested last week, would be prosecuted by the government in a public demonstration of opposition by right-wing extremism.

Diplomatic and Salvadoran government sources said in telephone interviews today, it appeared as late as last night that D'Aubuisson and eight of his closest associates would be brought to trial despite opposition from influential sectors of the Salvadoran military. Leaders of the Christian Democratic Party, which holds two of the five seats on the governing military-civilian junta, have said repeatedly over the last week that they would resign if D'Abuisson were freed. U.S. support for the government has been made contingent on their participation.

His release last night was the decision of an Army major, designated as special prosecutor for the case, who sources said consulted with the Salvadoran defense minister, Army Col. Guillermo Garcia. The reasons given were insufficient witnesses and evidence to hold D'Aubuisson more than 72 hours under Salvadoran law, despite the fact that a suitcase full of allegedly incriminating documents was found in D'Aubuisson's possession and nearly 30 people were arrested with him last week.

One senior diplomat reached by telephone in El Salvador called the release of D'Aubuisson "a case of lunacy. This is about as foolish a move as possible. It plays right into the hands of the ultraleft, which says the government is run by right-wingers anyway. This whole elaborate house of cards could start to shift. This is very bad."

An emergency meeting of the Christian Democratic Party to decide what future course of action its members will take continued throughout today. One senior party member indicated that there was still some support for the current government within the party, but that many members feel the time for compromise and participation is over.

D'Aubuisson's name often has been linked to right-wing terrorist groups responsible for many of the most brutal assassinations in the persistent war of atrocities waged by El Salvador's political extremists. Among those political figures believed murdered by right-wing extremists in recent months have been San Salvador Archbishop Oscar A. Romero and several ranking Christian Democrats. Others have been forced to leave the country.