Poland's Communist leadership, preparing to inaugurate a new legislature and Cabinet, today promised early action to release some political prisoners but indicated that well-known leaders of the banned Solidarity trade union probably would not be freed.

Government spokesman Jerzy Urban announced at a press conference that the government of Gen. Wojciech Jaruzelski would "soon" adopt a limited amnesty covering some of the country's 368 political prisoners.

Urban indicated, however, that the action would not be an amnesty approved by the legislature, but a more limited executive clemency that would not cover "repeat offenders," or persons who have been charged or jailed more than once.

This definition apparently would exclude several of the best known opposition figures under detention, including Adam Michnik and Wladyslaw Frasyniuk, two of the three Solidarity leaders whose trial and incarceration last summer drew strong protests from the Reagan administration and other western governments.

Today's announcement, the strongest government statement so far on a possible amnesty, came as the Communist Party Central Committee met to consider political strategy on the eve of the inauguration of the new Polish legislature elected last month. The opening of the assembly, the Sejm, is expected to be accompanied by changes in Jaruzelski's Cabinet and the initiation of a new phase of government political acitivity.

Tonight, reports circulated widely that Jaruzelski would give up his government post as premier during Wednesday's first session of parliament in favor of Deputy Premier Zbigniew Messner, a loyal supporter. Jaruzelski would retain his principal post as Communist Party first secretary. Diplomatic sources said the shift had been planned for some time but may have been speeded up in the aftermath of the legislative election, which the government labeled a success.

In a speech to the Central Committee broadcast on television tonight, Jaruzelski said that following the parliamentary elections, the government was "stronger, and so we can afford more energetic activity." While not mentioning the amnesty, he said "radical steps" were necessary to improve Poland's sluggish economy.

Although an amnesty measure has been viewed here and in Washington as a possible first step toward an improvement in relations between Poland and the United States, U.S. government sources have said a clemency excluding the top Solidarity figures would be insufficient to justify such steps as the lifting of U.S. sanctions against Poland.

A partial measure is also unlikely to satisfy government opponents, who this week have organized a "political prisoners' week" to press for a full amnesty.

There have been several other government moves suggesting a limited liberalization. The state-controlled press recently has printed several articles with unusually strong criticism of government performance or calls for political reforms, and last week the Communist-backed Patriotic Front for National Revival authorized the establishment of a new, nominally independent political organization with a mandate to "organize dialogue."

At the same time, the government has moved aggressively to head off a potential controversy over the death of a 19-year-old student detained by police for drunkenness Oct. 19 in the northern town of Olsztyn.

Reports by opposition sources have suggested that Marcin Antonowicz, who was picked up by an ambulance with a serious head wound minutes after his arrest, may have been the victim of brutality. Antonowicz died Saturday without regaining consciousness.

Urban, however, denied today that police were responsible.

Urban announced that the chief of Polish militia forces and a deputy prosecutor general had been dispatched to investigate the incident.