The Soviet Union reported without comment today the imposition of martial law in Poland after citing with approval the tough and sweeping measures announced by Premier Wojciech Jaruzelski earlier in the day.

But while the Soviets may be pleased that Jaruzelski adopted the course of action they had urged for some time, they seemed in their guarded dispatches to regard the Polish situation as very serious -- presumably because his failure to maintain control could mean Soviet military intervention.

There were no indications of any unusual activity in Moscow, which enjoyed almost springlike weather today after a bitterly cold spell. Western diplomats said there were no signs of any unusual military moves by Soviet forces near the Polish border.

The Polish crackdown precedes a meeting of Soviet Bloc leaders later this week. Soviet sources disclosed yesterday that the summit, which had been planned as a largely ceremonial gathering for Soviet President Leonid Brezhnev's 75th birthday Saturday, is now expected to turn into a major political event.

The official news agency Tass, in a report from Warsaw tonight, said the situation was "generally calm" there, in Gdansk and "most other areas of the country." It also reported in full Jaruzelski's speech to the nation, omitting only the final and emotional appeal to Poles.

Moscow radio and television broadcast the Tass dispatches, including Jaruzelski's pledge that Poland would remain "an unbreakable member of the socialist community."

The Soviet public was informed that martial law was imposed "in the face of the anarchy threatening the country and the irresponsible activities of extremist forces" in the Solidarity independent trade union.

Last night, a Tass dispatch described the situation as reaching a crisis stage, with Solidarity "preparing for direct seizure of power." The agency assailed "criminal actions" taken by Solidarity leaders at their Gdansk meeting.

Western observers said they believe the Polish authorities have advised the Soviet Bloc governments in advance of the intention to declare martial law. These observers said recent sharp Soviet press attacks may have been designed to dramatize the impasse between the union and the Polish government.

Last Tuesday, the Soviet armed forces daily compared the situation to that in Chile prior to the military coup that overthrew Socialist president Salvador Allende.

The tone of today's dispatches was distinctly lower key and Western analysts predicted the Kremlin would refrain from commenting until the course of events becomes clearer.

The timing of the crackdown seems to have been carefully calculated, with Poles coming into the hard part of winter and the Christmas season.

The prospect of repression by Polish authorities may entail some diplomatic and trade costs for Moscow. There is a general agreement among specialists here that the Soviets do not want to become militarily embroiled in Poland. Such a move would be too costly in terms of Moscow's relations with Western Europe and would make the Soviet Union virtually the sole responsible agent of economic support for a ruined Polish economy.

But, according to these specialists, the loss of Poland would also present an unacceptable setback for the Kremlin. Moscow is seen as ultimately prepared for short-term setbacks in its relations with the West should the situation in Poland deteriorate to a point where only a Soviet intervention could save the Communist government.

Western analysts here noted that a Tass summary of Jaruzelski's speech included his call for the "establishment of a front of national accord," suggesting a Soviet hope that the Polish government would be able to work out a compromise with moderate forces in Solidarity and thus avoid a confrontation. News services added:

In Eastern Europe, which has watched developments in Poland largely with critical eyes, the state of emergency imposed in Poland was applauded by Czechoslovakia and Hungary and reported elsewhere without comment.

In Prague, the state television interrupted a children's program to report the development and said the public would be kept abreast of further news.

In Hungary, the last East Bloc country where a state of emergency was declared -- during the uprising of 1956 -- the state radio carried excerpts from Jaruzelski's speech and a report from its correspondent in Warsaw quoting recent militant statements by Solidarity leaders to justify the crackdown.

"The fact that the Polish government has decided to find a solution by its own resources is a good thing," Radio Budapest said.

The only East Bloc country maintaining silence was independent-minded Romania, which largely has kept out of the anti-Solidarity chorus and urged that Poles be left to solve their own problems.

Yugoslav radio and television carried the full texts of Jaruzelski's speech, and Communist Party sources in Belgrade said the Yugoslav leadership considered the Polish situation "extremely grave and critical." Yugoslavia, which professes nonalignment, has stressed that there must be no foreign interference in Polish affairs.