Salvadoran opposition forces appealed publicly to the government yesterday to participate in "direct talks" which the opposition groups said could include a broad range of other parties, including foreign countries.

The idea was quickly rejected as "absurd" by Roberto d'Aubuisson, powerful head of El Salvador's constituent assembly.

The proposal, released here by a representative of the civilian-guerrilla opposition coalition, suggested that third parties agreed on by both sides provide their "good offices" to set up negotiations to end the two-year civil war. Francisco Altschul, a spokesman for the opposition coalition, said the participants could be governments or nongovernmental groups.

Meanwhile, the Salvadoran Defense Ministry announced that four leaders of the opposition coalition seized last week under mysterious circumstances are in military custody and will be tried as "leaders of terrorist delinquent groups."

The Defense Ministry statement released late Monday named eight people who were being held, but only four of them were among 11 labor and political leaders whose disappearances were announced last week by human rights groups.

State Department spokesman John Hughes said the Salvadoran government had announced that the eight would be "charged with conspiracy and sabotage" under state-of-siege decrees. He added that the "U.S. government will continue to follow these cases closely." The Reagan administration must certify in January that El Salvador is making progress in ending human rights abuses for U.S. economic and military aid -- totalling $320 million this year -- to continue.

At a news conference in Mexico City, Guillermo Ungo, leader of the Democratic Revolutionary Front, the civilian part of the opposition coalition, said the proposal was the first formal, written one the opposition has made to the Salvadoran government. Ungo's group is allied with the five guerrilla units that make up the Farabundo Marti National Liberation Front.

In the past, the Salvadoran government has rejected various proposals for negotiations, saying the guerrillas should not be allowed to gain at the negotiating table what they could not win on the battlefield. The United States has supported that position but encouraged the Salvadoran government to seek talks to find a way for the opposition to participate in a presidential election scheduled for 1984.

Referring to the opposition's proposal that third parties help to set up and "witness" the negotiations, a State Department official said, "One of our concerns and a concern of the Salvadoran government would be who the players are. No sovereign government is willing to give away its own sovereignty to external players, not knowing who they are. It is highly problematical."

The official said Washington was "encouraged" by the Salvadoran Defense Ministry's announcement that eight opposition leaders are in custody. In the past, many opposition figures who were seized under similar circumstances have either been killed or never reappeared. Congressional critics of U.S. aid to El Salvador and human rights groups say the Salvadoran armed forces and allied death squads are responsible for most of the killings of civilians .

Responding to charges that the government was responsible for last week's disappearances, U.S. Ambassador Deane R. Hinton said in San Salvador that "no one at the top" had ordered the detentions, United Press International reported. "Now there is a nice question regarding who gave the orders, but it did not come from the Cabinet level as far as I can judge," Hinton said.