The Washington-based Inter-American Commission on Human Rights assumed a major role today in negotiations with left-wing guerrillas holding 16 diplomats hostage and hopes rose here that the 58-day impasse was near resolution.

Commission President Tom Farer, who is a Rutgers law professor, and two other representatives of the seven-member commission met with guerrilla negotiators at the besieged Dominican Republic Embassy for the second time in two days. Two Colombian government officials accompanied them.

According to unofficial accounts, the M19 guerrillas modified their already reduced demands and indicated there could be a prompt end to the drama if the commission were to assure the fairness of two current military trials of some 250 accused guerrillas.

"I am confident of a quick solution," said Colombian Communications Minister Jose Arias Carrizosa. His government held 17 negotiating sessions in a van outside the embassy before the commission accepted a Colombian request to mediate. Initial demands for release of 300 prisoners have been whittled down until they reportedly have now been dropped altogether. h

Later, the commission met with Turbay after the Cuban ambassador to Colombia entered the embassy to assure the guerrillas that Cuba is prepared to offer them asylum. The guerrillas also held another sesson with government officials. The Cuban ambassador said a solution was imminent."

While the commission, an autonomous body of the Organization of American States, often has played an active role in assisting potential victims of political violence, this was the first known case of its taking a go-between role for a government confronting a rebellious opposition.

The governments intensely interested in the outcome include that of the United States, whose ambassador, Diego Ascencio, is among the hostages.

Accompanying Farer in today's session was Andres Aguilar, a Venezuelan who stepped down from the commission presidency because of the press of other commitments. Aguilar was one of the five members of the U.N. commission that made an abortive effort last month to resolve the U.S. hostage crisis in Iran. The other OAS commission member at today's session was Carlos Dunshee de Abranches, a Brazilian.

The commission's mediating position appears to be buttressed by the focus of attention here on the military trials of alleged guerrillas. Guerrilla groups have charged grave violations of human rights to the military, which has wide powers to suppress violent opposition and to try those accused of responsibility.

President Julio Cesar Turbay Ayala invited the commission to observe the current trials in hopes it will refute charges of defective justice. Among those making that charge has been the Nobel prizewinning rights group, Ammesty International.

Whether or ot the military trials prove protective of the rights of the accused, they produce a vivid image of courtroom confusion. The press is free to attend and did so today at an auditorium on a nearby military base that has been converted into a makeshift tribunal.

Col. Faruk Yanine Diaz paced on the stage, trying to keep track of the thousands of pages of testimony, the 41 alleged guerrillas seated before him and the 20 defense lawyers who interrupted the military prosecutor's train of thought every few minutes with legal questions he was uncertain how to answer.

Yanine appeared to struggle with his conscience each time one of the defendants rose to charge that he was tortured into confessing his alleged crimes. Although Yanine is a prosecutor, he is not a lawyer. He is trying 50 alleged members of the Armed Revolutionary Forces of Colombia, nine of them in absentia, on general charges of "rebellion" or more specific charges of ambushing a military convoy on Jan. 18, 1979, when seven soldiers were killed.

The trial began last November and is not expected to end until June or July.

On Tuesday, 15 of the defense lawyers, led by Humberto Criales de la Rosa, a Communist Party senator, presented a 10-point document to the OAS rights commission outlining legal and moral objections to the proceedings.

The document names 15 of the 41 defendants who claim they were tortured and says that most of the accused were illegally forced to testify against themselves and their relatives, were denied the right of counsel during interrogation and were questioned about their political beliefs in violation of Colombian constitutional guarantees.

Although President Turbay has denied that torture was used "as a general practice" against suspected subversives and has strongly defended the fairness of the trials now under way, Yanine -- confronted daily with defendants who claim that their bodies were subjected to electric shocks or their heads were repeatedly submerged in water during their interrogations -- does not seem so sure.

Yesterday, when another defendant described the scars on his body he claimed are the residue of the electric shocks, Yanine turned angrily to a representative of the attorney general -- who has the responsibility for investigating rights violations allegedly committed by civilian or military police.

If there were tortures, investigate them! I am the prosecutor but I have had nothing to do with torturing anyone. If there were tortures, investigate them!" he said.

Yanine later acknowledged, in response to a question by one of the lawyers, that "yes, the penal code says very clearly that if there was pressure [against defendants or witnesses] then their statements are invalidated. But I don't know how to determine this. . . . if there were tortures."