A leading defense attorney in a major trial of dissidents in Yugoslavia has been imprisoned and disbarred in a move that has drawn renewed western criticism of the government in Belgrade and prompted efforts by U.S. and international lawyers' organizations to gain his release.

Vladimir Seks, 42, was preparing the defense of a client about to be tried on charges of "hostile propaganda" when he was jailed Feb. 12 to serve an eight-month sentence for a 1981 conviction on similar charges in a case that is still under appeal.

Seks, in frail health with an ulcer and heart disorders, is now in a Zagreb prison hospital in the third week of a hunger strike, according to U.S. sources who have followed his case, and has been prohibited from seeing his wife or his lawyer. This week he was automatically disbarred although his own Croatian Bar Association has repeatedly issued statements in support of him.

In a rare public involvement in such matters, the American Bar Association on Monday called on the president of Yugoslavia to pardon Seks and asked Secretary of State George P. Shultz to use his influence to gain his release.

ABA President John C. Shepherd said he had told Yugoslav President Veselin Djuranovic and Shultz "that only a pardon by the presidency of Yugoslavia can spare that country the international outrage which will surely follow the death or continued imprisonment" of Seks.

Shepherd said the imprisonment of Seks "renders it impossible for his client to receive a fair trial and is a serious violation of international legal norms concerning the right of the accused to be represented by the attorney of his choice."

The State Department said yesterday that it had no comment on the case. Other sources said, however, that David Anderson, the U.S. ambassador in Belgrade, has been "working quietly" on Seks' behalf.

Amnesty International, which has been involved in Seks' case since 1981, and the Geneva-based Center for the Independence of Judges and Lawyers also have criticized the imprisonment of Seks.

A spokesman for the Yugoslav Embassy said yesterday that Seks had been "sentenced according to Yugoslav law" and his case was being handled with "normal procedures as with all" such cases. As of yesterday, the spokesman said, Seks was in "good health" despite his hunger strike.

Seks is the lawyer for Vladimir Mijanovic, one of six men charged last fall with conspiracy for a variety of alleged offenses that included private criticism of Yugoslav socialism and the late President Tito.

The cases of Mijanovic and another defendant were separated for later trial. The three-month trial of the other four ended Feb. 4 with one acquitted and three convicted of lesser charges, with sentences up to two years.

The trial stirred widespread criticism in Western Europe and the United States of Yugoslavia's apparent crackdown on political discussion. Similar criticism has been directed at the 1981 conviction of Seks, long a prominent attorney for dissidents in Croatia, one of the most hard-line Yugoslav republics.

Seks was accused of criticizing Tito in a bar and urging that the Yugoslav Army be structured along lines of the Hitler Youth brigades.

Of the chief prosecution witnesses, one was a convicted burglar and the other, according to Amnesty International, said later in a taped conversation that he had been told what to testify by security police.

Seks had alienated the secret police earlier, American sources said, when, as an assistant prosecutor, he had attempted to prosecute some of them for illegally opening mail. The day before his 1981 arrest, he had irritated police, sources said, by visiting a client -- a Yugoslav nun -- jailed on charges of "engaging in anti-Yugoslav propaganda" while she was visiting relatives in Austria.

A Croatian court initially sentenced Seks to one year, but a federal court, citing "procedural irregularities," overturned the verdict. In a retrial, the local court sentenced him to eight months, removing the case from the jurisdiction of the federal court.