Just 18 months ago, Maciej Szczepanski was one of the most powerful men in Poland. As head of state radio and television, he acted as chief image maker for the former Communist government--covering up its disastrous economic mistakes with what became known as "the propaganda of success." The ex-chief of the Communist Party, Edward Gierek, described him as "my favorite son."

Today Szczepanski, along with four of his closest associates, went on trial in a Warsaw court accused of a series of criminal acts ranging from embezzlement to accepting bribes to falsifying documents.

Set against the greater drama of martial law, the Szczepanski trial may seem a minor episode. But it is important, not only because it is the first trial of former senior Communist Party officials in a Soviet Bloc country for many years, but also because of the insights it gives to the way Poland was run in the 1970s under Gierek.

The indictment against Szczepanski alone runs into 600 pages--a catalogue of alleged abuses committed during 10 years of Gierek's rule. The 53-year-old former television chief had earlier been accused in the Polish press of diverting state money to companies he secretly owned in Britain and Liechtenstein and using public funds to import some 900 pornographic films which he lent out to high-ranking friends.

The trial was originally scheduled to open Jan. 14 but, following the imposition of martial law last month, it was speeded up. Poland's military authorities have sought to create an impression of even-handedness by moving against both the independent Solidarity trade union movement and discredited former Communist Party officials at the same time.

A large television broadcast van was parked in the courtyard alongside police trucks to beam pictures back to the service Szczepanski used to run like a private fiefdom.

Blinking under the powerful television arc lights, Szczepanski entered court in an open-neck shirt and shabby dark suit--a marked contrast to his smart appearance during his days in power. He exchanged private jokes with his former deputy, Eugeniusz Patyk, who is also accused of accepting bribes from foreign companies and illegal dealings in hard currency.

What Polish newspapers have described as Szczepanski's "monarchical style" of life has become legend since his dismissal at the height of the labor unrest in August 1980 that gave birth to Solidarity. Lurid tales have circulated about a private harem and he allegedly had many residences including one with an outdoor pool.

The defense, however, is expected to argue that the luxury villas frequented by Szczepanski were owned not by him but by the Committee for Radio and Television. Questioned by the judge today, he said he only had one car, an apartment in town and a country house.

Szczepanski, a former member of the Communist Party's policy-making Central Committee, had a minor skirmish with the judge who twice asked him to give his middle name. "I only have one name Maciej--as far as I remember, I was not given another one," he insisted.

Like Gierek, Szczepanski came from the southern industrial region of Silesia and was one of the most influential members of what became known as "the Silesian mafia"--the group of politicians that ruled Poland. One of the charges against him is that he used public funds to buy a gold broach which he gave to Gierek as 65th birthday present.

Many of Szczepanski's former subordinates at the radio television headquarters are expected to be among more than 200 witnesses called for his trial.