They are the eyes and ears, and the hidden muscle, of the regime. They move in shadows, dress in plainclothes, eavesdrop through walls and phone lines, as if pretending not to be there.

But everyone knows, and is supposed to know, that they are around -- and to fear them.

They are presumed to be masters at dirty tricks: at break-ins, faked accidents, mysterious deaths. They keep thick files on people. They torture, or so many have alleged, like the four secret police officers now on trial for the murder of the Rev. Jerzy Popieluszko.

Every dictatorship has them, is sustained by them. In Poland, they are called the Sluzba Bezpieczenstwa, or security service.

They are a privileged group, better paid on the average than others of similar age and background. They jump long waiting lines for cars and apartments, shop in favored stores, vacation in reserved spots and receive medical treatment in a Warsaw hospital known to have better care and more hard-to-get drugs than other hospitals.

They are relatively intelligent. A high percentage are college graduates. They also tend to be loyal members of the ruling Polish United Workers Party.

They do not like publicity, which is why they are not happy about the trial in Torun against Popieluszko's killers. Never before in the history of communist Poland have the inner workings of the secret police been so exposed to a public spotlight.

One result has been a certain debunking of the security service's mystique. The team that murdered Popieluszko comes across as bumbling, panicky and ill-equipped. They drove a car that leaked oil. They made clubs out of broken fence posts, scrounged for an extra pair of handcuffs and used an old towel and T-shirt as gags in the kidnaping. They could not prevent Popieluszko's driver from escaping, and they couldn't make the priest immobile in the trunk without beating him to death.

On the other hand, the way these officers thought was as chilling as outsiders imagined. Their mindset was starkly expressed in court by ex-captain Grzegorz Piotrowski, the accused chief plotter. His intolerance of opposition views, his anger at government concessions and, most significantly, his arrogant sense of operating above the law, shielded, he thought, from the consequences of illegal action, are confirmation for many Poles that the security service exists isolated from the sentiments and standards of the rest of Polish society.

The trial of the four officers has challenged this once immutable principle of unpunishability. Some Poles are convinced that the court action has saved the lives of many others who would have fallen victim to a wave of secret police attacks had Polish leader Gen. Wojciech Jaruzelski not moved against Popieluszko's killers. Others believe that the trial's restraining effect on the secret police will be short-lived and that a new crackdown on underground operations may be ordered soon to assuage the police establishment.

Mindful of the need not to chip away too deeply at a pillar on which they stand and rule, Polish officials have been characterizing the four on trial as exceptions, not -- as many Poles see them -- as examples of a basic and dangerous rot in the system.

Just how many secret policemen operate in Poland is, like most everything else about them, classified information. The London-based International Institute for Strategic Studies, which tries to estimate the size of military and police forces around the world, lists Poland's internal defense troops at 65,000 and its citizens' militia at 350,000. Somewhere in those estimates is included a rough count of the strength of the security service, but institute spokesmen say they are unable to give a separate number.

Whatever the figure, Polish sources say it has been on the rise since the labor upheaval of 1980 that produced the Solidarity trade union movement and the spread of political underground structures after martial law was declared at the end of 1981. An article on secret police activities that appeared last summer in the underground journal Baza said that in Krakow alone, the number of regular security service employes jumped 25 percent in recent years, from 1,600 to 2,000.

In addition to full-time agents, there are thousands of informers and collaborators whose ranks extend the reach of the security apparatus into many corners of society. Even so, the service, in one of its most embarrassing failures, for three years has been unable to capture Poland's most wanted political fugitive, Zbigniew Bujak, former Warsaw region Solidarity leader and now head of the main Solidarity underground coordinating council.

The Baza report also described the security service's structure. It said there is an operations division split into departments responsible for monitoring western intelligence activities in Poland, economic espionage, the press and intelligentsia, industrial and worker circles, the Roman Catholic Church, farmers and the hard-core opposition. There is also an investigations division and one for "auxiliary services" which carries out surveillance, break-ins, forging of documents and other tricks of the trade for the operations division.

The Torun trial has provided a window into a few of the service's underhanded methods. One of the defendants, ex-lieutenant Leszek Pekala, said that a mysterious fire last October in the apartment of Popieluszko's driver, Waldemar Chrostowski, was part of the government's clandestine operation against the priest. Piotrowski testified that Interior Ministry officers also had discussed placing a flammable chemical on the exhaust pipe of Popieluszko's Volkswagen to cause a fiery explosion.

When abducting Popieluszko, one of the agents was dressed as a traffic policeman. The kidnapers also considered posing as underground activists irate over the priest's alleged embezzlement of Solidarity union funds.

Not only did the disguise themselves, but they also put stolen license plates on their Polish-made Fiat. Piotrowski told the court that the Interior Ministry maintains a stock of civilian license plates traceable only to the ministry's address on Warsaw's Rakowiecka Street. But in the interest of even greater secrecy, the kidnapers did not draw on this stock.

What fascinates many Poles, too, are references in the testimony to the special privileges and powers enjoyed by the security service. Pekala acknowledged, for instance, that he probably did not need a special clearance pass issued by his superior for the kidnaping mission to avoid militia road checks, which are commonplace in Poland. He said he is usually waved on unsearched by militia patrols after flashing his ministry identity card.

Piotrowski confessed at another point in the trial that he often accelerated the issuance of travel passports for people, though he denied allegations that he had taken bribes for doing this.

Like other military or paramilitary organizations, the security service lives by codes of loyalty, secrecy and strict obedience to authority. But like groups obsessed with producing and combating conspiracies, it is prone to intrigues in its own ranks.

Piotrowski testified that his boss, Col. Adam Pietruszka, accused as an accomplice in the murder, relished setting up a "conspiracy within a conspiracy," pitting the local Warsaw internal affairs office against a section in the service's national headquarters.

It is evident from the trial that the interior minister, Gen. Czeslaw Kiszczak, who oversees the secret police along with Poland's other police forces, was not in firm control when Popieluszko was killed. As an Army intelligence officer transplanted to head the police apparatus in 1981, Kiszczak is widely said to be resented by some career agents at the ministry. He is a close associate of Jaruzelski, and thus supportive of the policies of amnesty and accommodation with the Roman Catholic Church that are in disfavor among some security service officers.

At the moment, Poland's secret service is undergoing a review of personnel. A new deputy interior minister has been appointed -- a reputed Communist Party disciplinarian named Andrzej Gdula -- and he is expected to lead the shake-up. It is also rumored that he may replace the service's current chief, a two-star general and deputy minister named Wladyslaw Ciaston.