The White House declared yesterday that the Soviet Union appears to have completed preparations for possible military intervention in Poland and underscored its earlier warning to Moscow that such an action would have grave consequences.

The statement capped a day of mounting tension here and abroad. President Carter summoned his top national security advisers to meet with him twice over the brewing crisis in Poland and then called in congressional leaders for a special White House briefing.

Behind the heightened concerns here were several potentially ominous developments, according to informed U.S. officials.

These sources said that there was now "a lot of movement" involving Soviet and other Warsaw Pact military units throughout communist Eastern Europe.More Soviet divisions have "come out of their garrisons" in the western districts of the Soviet Union closest to the Polish border. More Soviet reservists have been called up -- at an accelerated pace -- in the last few days. Command and communication facilities linking military headquarters in the Soviet Union to other headquarters in East Germany and Czechoslovakia, which also border on Poland, have also been brought to the top state of readiness. These command posts would coordinate any intervention that involved Soviet forces in those three countries along with separate Czech and German units taking part.

Though all of these measures are aimed at Poland, the very slight chance that they could mean something even worse -- a spillover into Western Europe -- has also prompted the Pentagon, according to high-level sources, tol order "some modest contingency preparations." These do not include alerting U.S. troops but do involve placing certain military headquarters in Europe on a higher alert status.

Officials continued to stress, however, that there was nothing the United States or its allies could do militarily to affect the Soviet-Polish situation and there would be no U.S. military involvement.

The White House actions yesterday also came on the heels of what informants say was a grim, just-completed U.S. intelligence estimate that the Soviets apparently concluded late in November that some form of "coercive action" would be necessary to stem the rising challenge to communist authority in Poland.

White House officials said yesterday they were still in no position to say that a Soviet military move against Poland was imminent and others said they were not absolutely sure what Moscow's intentions were. But there was wide agreement that the assessment of the situation here had turned increasingly pessimistic in recent days.

There was also no certainty about what kind of action the Soviets might take, but a number of officials said the Soviet intervention might not come, or start out, as a full-fledged invasion but rather might come in the guise of a joint military exercise involving Warsaw Pact forces.

Intelligence estimates here reportedly indicate that if the Soviets should intervene, they might do so this month. Two things, in particular, are thought to be of concern to Moscow. One is the anniversary, on Dec. 16, of worker riots in 1970 in the port city of Gdansk that eventually toppled the government of Wladislaw Gomulka. The second is a planned review by the independent labor unions of reforms promised by the government after last summer's strikes in that same city.

In Gdansk yesterday, it was reported that about 300,000 peoples massed outside the shipyard that was the scene of last summer's strike and officials estimated that possibly 1 million people may attend the dedication of a workers monument there on Dec. 16.

One major factor that would seem to argue against an immediate intervention is Soviet leader Leonid Brezhnev's visit to India, which begins today. This is Brezhnev's first visit to a non-communist country since the invasion of Afghanistan one year ago and some analysts believe he would not choose to be away if a crucial military operation were about to be launched. Others, however, think the Brezhnev trip might be meant to play down any joint military operation in his absence.

Carter met yesterday morning with a small "crisis management" committee headed by national security adviser Zbigniew Brzezinski. Later, the president met for more than one hour with the top-level National Security Council, including Secretary of State Edmund S. Muskie and Defense Secretary Harold Brown.

Also called to the White House for a special 20-minute briefing were House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), House Minority Leader John Rhodes (R-Ariz.), Senate Minority Whip Ted Stevens (R-alaska) and Senate Majority Whip Alan Cranston (D-Calif.).

At a hastily called news briefing later, the White House issued a terse statement that "preparations for possible Soviet intervention in Poland appear to have been completed. It is our hope that no such intervention will take place. The United States government reiterates its statement of Dec. 3 regarding the very adverse consequences for U.S.-Soviet relations of Soviet military intervention in Poland."

A senior White House official, who asked not to be identified, said the United States has been "monitoring carefully" the Soviet preparations, and that they involved "logistical and deployment activities. They are ready to move," he said, though repeating that he was in no position to judge exactly what the Soviets would do. The preparations, he noted, had continued after Carter's warning of Dec. 3.

Other officials said privately that until this weekend, there were still a few things that analysts here believed the Soviets would have to do to get into position to intervene. Now all that is done, and the Soviets essentially could now move with no warning time for Poland or the West.

The White House official said that President-elect Ronald Reagan's transition team has been kept up to date on the situation. Reagan's chief foreign policy adviser, Richard V. Allen, said on "Issues and Answers" (ABC, WJLA) that Carter obviously remains fully in charge of U.S. foreign policy, and that Reagan does not want to interfere and will back Carter in the defense of U.S. foreign policy. Allen repeated the Republican warnings about the "profound repercussions" such a Soviet move would have.

The public White House warnings were denounced by Moscow last week as contributing to tensions. Some American officials also believe the United States may have spoken out too frequently in recent days, as the crisis appeared to run both hot and cold. Last Wednesday, for example, a senior White House official told reporters that a Soviet intervention was not imminent. But yesterday he said "as of now, we are not in a position to say whether it is imminent or not."

Other officials felt that the Carter administration, which failed to warn the Soviets publicly before they intervened last year in Afghanistan despite clear signs of a military buildup, was now being doubly sure that its concerns would not be underestimated in Moscow.

White House officials also called attention to the statement's use of the word "intervention" rather than "invasion," a difference that apparently suggests the belief that the Soviet military movement into Poland in greater force could come through some form of an exercise.

The Soviets have two divisions based inside Poland, another 20 in East Germany, five in Czechoslovakia and 33 divisions in the western districts of the Soviet Union. It is estimated that Moscow might need 30 divisions and upwards of 300,000 troops if a major invasion were to take place and the 200,000-man Polish army, or portions of it, were to resist.

In the aftermath of a Warsaw Pact summit meeting in Moscow last Friday, a communique issued by the communist leadership appeared to signal that the Kremlin was prepared to give Poland's new political leadership more time to work things out.

Some U.S. officials still believe this is the case and they got support yesterday from former West German Chancellor Willy Brandt, who appeared on the program "Face the Nation" (CBS, WDVM).

Brandt said he felt that "at least an important part of the Russian leadership would hate to intervene." He suggested that the Polish military, if convinced that the situation at home was getting out of control and the Soviets were coming, would try to act and restore order in Poland in order to remove the need for Soviet troops.