A Yugoslav court gave relatively light sentences today to three Belgrade intellectuals convicted on a charge of "hostile propaganda" in a political trial seen as having mixed implications for the cause of free expression here.

At Belgrade's Modern District Courthouse, in a large chamber adorned only with a portrait of the late Yugoslav president Tito and crowded with local and foreign observers, a chief judge ordered prison terms of two years for scriptwriter Miodrag Milic, 55, 18 months for sociologist Milan Nikolic, 37, and one year for radio station publicist Dragomir Olujic, 36.

The sentences were milder than usual for similar cases in Yugoslavia and Eastern Europe generally. Defendants and their supporters attributed this to publicity that the proceedings had generated in the West, as well as to differences within the Yugoslav Communist leadership over the necessity of the trial.

But the convictions upset Belgrade's active intellectual community, whose members packed the 300-seat courtroom to hear the verdict, then stomped out in protest as the judge began to read the justification.

"The message is: you can't think any other way than what is officially accepted," declared Nikolic's attorney, Tanya Petrovar.

Other Yugoslavs, while agreeing that the court action represented a blow to free and pluralistic expression, said the short prison terms marked a setback for party hard-liners who initially had pushed the case. Failure of the prosecution to win a conviction on an initial conspiracy charge was seen as likely to encourage the resumption of private discussion groups, whose meetings were interrupted after a raid on one last April led to the trial.

In fact, several dozen intellectuals gathered in a Belgrade apartment 10 days ago to restart the "free university," a series of discussions touching on diverse topics begun eight years ago and in which the defendants participated.

"The purpose of the trial -- that is, to intimidate intellectuals -- was not achieved," proclaimed Mihailo Markovic, a prominent dissident professor of social and political philosophy. As one diplomat summed it up: "The authorities have retreated some. But it doesn't mean the cause of human rights has been advanced."

The three convicted today went on trial Nov. 5 with three others, all originally accused of the more serious charge of using the free-university sessions to undermine the system and overthrow the existing government. Two of the initial six -- Vladimir Mijanovic, 38, an unemployed sociologist, and Gordon Jovanovic, 24, an art student -- were separated from the case last month on procedural grounds and will be tried separately.

The prosecution, acknowledging a lack of proof to make the conspiracy charge stick, reduced the charge for three defendants to hostile propaganda and dropped action against Pavlusko Imsirovic, 36, a free-lance translator.

The retreat was interpreted at the time as reflecting misgivings among the Yugoslav leadership about the trial's damaging impact on the country's international reputation as a tolerant communist state. Even some hard-line party officials -- among them, Stipe Suvar of Croatia, who called the trial "unnecessary" -- began to dissociate themselves from the proceedings.

That the case was brought at all had been explained earlier as an attempt by hard-line elements, particularly in Croatia and Bosnia, to pressure a more moderate Serbian leadership to curb the growth of dissent since Tito's death in 1980.

Pending appeals, the defendants are to remain free, as they have been during the trial.

The court based its decision on a few texts found in police searches of the defendants' homes. The papers, said the accused, had been written for personal use. One cited in the verdict was a paper on the structure of Yugoslav society prepared by Nikolic as part of his master's thesis at Brandeis University.

Chief Judge Zoran Stojkovic said the texts contained false and insulting references to the Communist Party and to Yugoslav institutions and, in some instances, defamed Tito. He rejected claims that the works were essentially scientific, labeling them political tracts.

The judge portrayed the defendants as inspired by some of the country's most notorious dissidents, mentioning Croatian writer Mihailo Mihailov and Milovan Djilas, formerly one of Tito's closest associates.

In a justification of the verdict that read like a political defense of the Yugoslav system, the judge chided Nikolic for violating a patriotic obligation not to denigrate one's country when abroad, adding that in America particularly, where Nikolic studied, many people are unfamiliar with conditions in Yugoslavia -- or even its location.

"There are weaknesses in our country, but the party and the authorities are doing their best to rectify them," said the judge.

He was interrupted repeatedly by the defendants, who sat facing him in the first row of the courtroom. Imsirovic, though freed, was seated with the defendants and led the walkout, shouting over his shoulder: "I'm leaving because this is the same as a party meeting."

Shortly afterward, the judge ordered out of the courtroom a heckling Milic, who retorted: "Thank you. I never wanted to stay anyway."

The exchanges were indicative of the freewheeling character of the trial. Normally, political trials in Eastern Europe are closed-door, cut-and-dried affairs. A loosely structured, bazaar-like atmosphere prevailed, typical of many attempts in the multinational society of the Balkans to balance competing forces.