Testimony has begun in a case that will decide whether the Immigration and Naturalization Service has a duty to inform every El Salvadoran illegally in this country of their right to apply for political asylum.

The class-action suit, filed on behalf of all Salvadorans in the United States, is being argued in U.S. District Court here.

In 1982, U.S. District Court Judge David V. Kenyon issued a preliminary injunction that is still in effect, ordering the INS to inform all Salvadorans of their right to ask for political asylum. In this case, the plaintiffs have asked that the preliminary order be made permanent and that improved conditions be provided for illegal aliens being held in detention, including easier access to attorneys and telephones.

In his opening arguments earlier this week, attorney Mark Rosenbaum of the American Civil Liberties Union said that, while the preliminary injuction has done "much good," many immigration officials have ignored the order and advised Saladorans not to apply for asylum.

Government attorneys, led by Allen Hausman, assistant director of immigration litigation for the Justice Department, made no opening statements. Outside court, they told reporters the government opposes the permanent instruction, noting that it would add to the the backlog of immigration cases. The government estimates that some 25,000 Salvadorans in the Los Angeles area alone are scheduled for asylum hearings.

The government argues that illegals who fail to express a fear of returning to their native home do not need to be routinely advised of their right to asylum. Rosenbaum contends that Salvadorans do not always understand the words they have to use. Testifying Friday through an interpreter, Salvadoran refugee Miguel Avila-Ochoa, 20, told of being held at a deportation center in El Centro, Calif., where he was pressured by INS officers to sign a voluntary deportation order as his lawyer was appealing his case.

Avila-Ochoa said immigration officials told him "I'd better sign, because the judge had already given me the deportation order, and that no lawyer in the world would be able to do anything about that . . . . They called in the general director of the center and he told be the same thing, I'd better sign because there was nothing else for me to do."

Avila-Ochoa said that INS officials said they would "put me on a bus and drug me or handcuff me -- use force." He said he was then put in a "punishment cell."