Poland's military leader, Gen. Wojciech Jaruzelski, today announced the release of a majority of the country's political internees and promised to suspend martial law by the end of the year if social peace is maintained.

His announcements were coupled with news of a postponement in the visit to Poland next month by Pope John Paul II. The communist authorities and the Vatican have agreed that the visit will not take place until next year.

Lech Walesa, head of the independent trade union movement Solidarity, is not among the detainees being released.

Jaruzelski, who holds the posts of prime minister and Communist Party leader, told the Sejm (national assembly) that communication restrictions between Poland and the outside world would be eased and more Poles allowed to travel abroad. Yet he made clear that his regime intends to maintain a tight grip even after martial law is lifted and will impose strict conditions on the resumption of trade union activity.

The measures outlined by Jaruzelski today were apparently designed to convince supporters of Solidarity, which was suspended with the imposition of martial law, of his good intentions and encourage Western governments to lift economic sanctions against Poland. They were timed to coincide with the anniversary Thursday of the formation of Poland's first communist administration at the end of World War II.

After Jaruzelski's speech, officials revealed that 913 persons interned under martial law, including all women, would be freed while a further 314 would be allowed home on parole. This leaves 637 persons still interned, including Walesa and most of his colleagues on Solidarity's decision-making national commission.

The first reaction of Western diplomats was to welcome the release of the internees but express concern at the lack of progress toward negotiation between the government and Solidarity leadership. They also noted that Jaruzelski failed to fulfill hopes of a partial amnesty for people charged with martial law violations.

As many as 2,000 Solidarity activists have either been convicted of such crimes as organizing strikes or distributing leaflets or are awaiting trial.

Today's parliamentary session, the last before the summer recess, followed a Communist Party Central Committee meeting last week at which Jaruzelski succeeded in removing one of his most powerful potential rivals, Stefan Olszowski, from the key post of Communist Party secretary in charge of propaganda. As expected, Olszowski was today named foreign minister--a post he occupied from 1971 to 1976.

The latest relaxation in martial law bore the hallmarks of the cautious political style Jaruzelski has developed since his military takeover last December. He has combined gradual relaxation of the harsh security measures imposed then with hints and promises of concessions to come provided the nation behaves.

On this occasion, the strongest incentive to good behavior is his announcement that the government will do all it can to facilitate a visit by the pope to his homeland next year. He added, however, that an "indispensable" condition for the visit was social peace in Poland.

A similar announcement on the postponement of the pope's visit was made in Rome by the primate of Poland, Archbishop Jozef Glemp. Glemp said the pope had taken the decision after considering "all the possibilities but also all the obstacles."

The pope had wanted to attend celebrations marking the 600th anniversary of Poland's most sacred religious image, the Black Madonna of Czestochowa, on Aug. 26. the communist authorities, however, recalling the emotional fervor stirred up by his first trip to Poland in June 1979, urged him to reconsider.

Delicate negotiations to reshedule the trip reached a climax earlier this week with a surprise mission to Rome by former foreign minister Jozef Czyrek. The most probable date now is next May.

The government's use of the papal visit as a carrot for Solidarity supporters was paralleled by Jaruzelski's promise to end martial law by December. But he made clear that this too would depend on the maintenance of social calm and that, even if martial law were suspended, the government would seek "special powers" for a further temporary period.

Dressed in the summer uniform of a four-star general, Jaruzelski insisted that martial law was only a means to an end. He said progress toward lifting restrictions had been slowed down by the hostility of Western governments, notably the United States, and underground resistance by Solidarity activists.

"Poland stands at a crossroads . . . .The time has come to choose between a road to nowhere and a road to building a constructive future for our country," he said.

Jaruzelski gave few hints about the future of Solidarity, other than insisting that trade unions must be revived and would be left "independent of the state administration." Later, however, a deputy prime minister, Mieczyslaw Rakowski, made clear that Solidarity would have to agree to radical changes in both its aims and organizational structure as a precondition for its reinstatement.

Reporting on the work of a government committee on the future role of trade unions, Rakowski said Solidarity's national commission was full of people openly opposed to the socialist system, but welcomed what he described as the readiness of some Solidarity activists to compromise. He said a social coordinating commission would be set up to prepare conditions for the rebirth of trade unions.

Among the "proposals" that Rakowski said had been submitted to his committee were a proposed ban on strikes until 1985, the outlawing of political and sympathy strikes, and the dismantling of Solidarity's regional structure. Rakowski said the new unions should be formed on the basis of individual professions--a move apparently designed to undercut the power of strong regional bodies.

In his speech, Jaruzelski said the restoration of trade unions would be preceded by the reactivation of workers' self-management in industry "within the coming three months".

Introduction of self-management in factories would be significant as it would help decentralize the economy--a necessary precondition for economic reform. The problem, however, is that most self-management councils, which were suspended under martial law, are controlled by Solidarity supporters.