The public prosecution that began last week of four secret policemen in connection with the killing of a Warsaw Roman Catholic priest, Jerzy Popieluszko, runs on several levels, being at once a trial for murder, an indictment of Poland's security apparatus and a test of a communist system that has fostered in some privileged officials a feeling that they operate above the law.

More than just a criminal hearing, the court process has already begun to reveal normally hidden machinery of the Interior Ministry, which controls Poland's police, focusing on that gray area of operations not strictly defined by law.

Not since a brief period during the mid-1950s, just after Poland threw off Stalinist repression, have Poland's secret police been held accountable in court for overstepping the law.

Public disgrace of members of a main pillar of the state is unusual in a country of the Soviet Bloc, as is the open nature of the proceedings. A limited number of western correspondents is attending.

Even so, many Poles remain skeptical that the perpetrators will suffer much in the long run or that the trial will bring basic changes in the operations of the internal security services.

The court proceedings are expected to make a fascinating show. But people here, made cynical by 40 years of what they often describe as communist manipulation, question whether it will come to more. They cite the pragmatic considerations of those in power. The trial is to resume Wednesday.

"It is a play," remarked a veteran Polish journalist, one of the country's most celebrated authors. "There may be some revealing disclosures about how the secret police work, how actions get planned and executed. This is the price the regime has to pay for the crime.

"But we will probably never know who was behind the murder. These people don't leave clear traces. Direct orders are never given. Besides, the government cannot afford to push too hard to get at those really responsible."

How difficult it is to pin down responsibility in an apparatus skilled in coded gestures and subtle directives began to come clear in the first two days of the trial.

The indictment tells of a number of meetings between the accused -- former lieutenants Leszek Pekala and Waldemar Chmielewski, former captain Grzegorz Piotrowski and ex-colonel Adam Pietruszka -- at which a plan of action against Popieluszko was discussed.

No one is reported to have ever stated explicitly at these sessions that Popieluszko must be killed.

Other expressions were used, mentioning the need to "eliminate" the priest's illegal activities and to cause his "disappearance." Such language sounds less incriminating than an outright instruction to murder, but the words seemingly were understood as euphemisms for it.

Already, too, Pekala, the first to testify, has begun to back away from pretrial testimony implicating a deputy interior minister, who was initially said to have ordered or known about the mission to get rid of Popieluszko.

In court Friday, Pekala said he had misunderstood something Piotrowski said about the deputy minister supporting an action. He told the court he now realizes his earlier impression of there being "highest level" backing for the assault was an "illusion."

Customarily, Piotrowski, who is listed first in the indictment, would have taken the stand at the opening of the trial. But Pekala, who is listed second, preceded him. Some lawyers in the case say this is because he was considered most talkative.

Why he has decided to soften his testimony can only be guessed at. But some Poles assume he hopes or expects to be spared if he sticks to certain limits in what he discloses in court.

That the trial is happening at all is widely attributed to the escape of opieluszko's driver, Waldemar Chrostowski, who fled the kidnapers by leaping handcuffed from their speeding car and sounding an alarm that eventually unraveled the crime.

Faced with the threat of a national uprising over the brutal slaying of the popular, pro-Solidarity cleric, the government had little choice but to search out and prosecute the priest's killers. It was also under pressure from Poland's influential Roman Catholic Church to do something and perhaps was mindful of what the western reaction would be.

In addition to these external concerns, Polish leader Gen. Wojciech Jaruzelski is thought to have agreed to the trial in view of an internal power struggle. He and his senior aides seem to have immediately interpreted the killing as part of a scenario to upset the general's leadership in favor of a harsher rule.

While no high-level officials have yet been linked to such a political provocation -- if that is what the murder was -- Jaruzelski has used the shock of the killing to attempt to seize firmer control of the sprawling security apparatus. He ordered a personnel review and called for tighter party supervision of the Interior Ministry.

Against this background, the trial serves Jaruzelski's interests by providing a dramatic lesson to any opponents who might be contemplating further provocations. But he must be careful not to do much that would upset the security fraternity that remains his main instrument of control over the country.

Nonetheless, the fact that Jaruzelski, a military man, would arrest his own policemen and threaten them with a death penalty in a public trial is extraordinary. Other communist rulers might have tried a cover-up or a trial under a veil of secrecy justified on grounds of national security.

This Polish distinctiveness is in line with such other variations here on the Soviet model as the strong position allowed the Roman Catholic Church, the dominance of privately owned farms in place of large agricultural collectives, a relatively lenient passport policy that permits more Poles than other East Bloc citizens to travel to the West and a limited tolerance of underground political activity.

The press is the least controlled, although it is still heavily censored, in the Soviet Bloc.

The fact of the trial does not seem to have won much sympathy here for Jaruzelski, who still is widely detested for crushing the independent Solidarity union movement three years ago. But it may have gained some understanding of his efforts for amnesty and accommodation with the Catholic Church, and of the opposition he faces.

How the other Eastern European states will choose to report on the unusual trial remains to be seen. Their correspondents have been granted seven places in the courtroom, the same number allotted western reporters. Most of the 100 seats are taken by members of the security apparatus that in a sense is facing the trial of its life.