The White House and the incoming Reagan administration bluntly warned the Soviet Union yesterday that Russian military intervention in Poland would have the gravest and most widespread consequences.

Reporters called in unexpectedly to a White House briefing by presidential press secretary Jody Powell late yesterday afternoon were told that "one thing they [the Soviets] can know with certainly is the nature of the consequences which would flow if there is intervention in or invasion of Poland.

"They would be most serious and adverse," Powell said, "both for East-West relations in general and in particular for relations between the U.S. and Soviet Union."

Powell said the situation had been discussed with advisers in the incoming administration of President-elect Ronald Reagan, adding that "it would be a serious mistake for any government, for any nation, under any circumstance to assume that a period of transition between one administration and the next . . . that the American government would lack either the wall or the ability to respond appropriately." Powell did not say what the U.S. response might be.

Earlier in the day, the senior foreign policy adviser to Reagan, Richard V. Allen, said on the NBC-TV "Today" show that "the consequences of an invasion would be severe and long-lasting. It could border on wrecking relations for a long time. I trust this will not happen."

At the same time, other U.S. officials said privately that Soviet efforts over the past three months to improve the combat readiness of their forces stationed near Poland have progressed to a point where the warning time that Poland, or the West, would have of such an attack is now less than a week.

The view is widespread among senior U.S. officials that the Soviets are fully aware of the extraordinary costs they would pay if they did invade Poland, that Moscow clearly would prefer not to and would do so only as a last resort if convinced that communist control over Poland was about to be lost. Senior officials say there is no evidence that Moscow has made a decision to intervene militarily.

Nevertheless, the view is also widely held among senior U.S. officials and foreign diplomats that if the Soviets become convinced that the continuing confrontation between the newly independent trade unions and the communist government in Warsaw is getting completely out of hand and government control is about to be lost, then Moscow will move military if necessary.

It is estimated here by U.S. specialists that the Soviets would have to count on resistance by the Polish population and possibly by the Polish army, and that Moscow would need at least 30 divisions, more than 300,000 men, in such an invasion force, a figure that dramatizes the potential scope of a confrontation.

Powell called reporters' attention to the strong statement by the leaders of the nine-nation European Economic Community yesterday that also warned Moscow of the "very grave consequences" of any would-be intervention and said Carter has been in personal touch with several allied leaders in recent days.

At the State Department yesterday, Soviet Union Ambassador Anatoliy F. Dobrynin was also summoned for an unannounced, hour-long meeting with Undersecretary of State David Newsom during which the Polish situation was discussed.

European diplomats here and some senior U.S. specialists said privately they believe there is increasing fear in the Soviet Union and in the communist governments of the other East European members of the Warsaw Pact that the new Polish government of Stanislaw Kania will not be able to cope with the continuing strains of spreading political demands being made by the unions. It is clear from denunciations of the union activities in the official press of East Germany and Czechoslovakia that the entire pact sees "the danger of infection" and similar unrest spreading through those countries.

One U.S. specialist yesterday described the situation in Poland as one of rapidly deteriorating government control and "near anarchy" in the political sense. Another said that Kania, who is trying, through compromise and warnings, to control the situation, and labor leader Lech Walesa "are both riding volcanoes."

Feeding the fears here and abroad is the unpredictability of almost everything associated with the crisis. There is the possibility that Kania can ride out the situation, or that the Soviets might try to install new political leadership before attempting military action. There is the possibility of severe splits within the Polish Communist Party between compromisers and those who want to confront the union leaders. This could lead to a harder-line faction taking power and "inviting" the Soviet forces into the country.

The Soviets' steady military maneuvers and other actions meant to improve the readiness of their troops alarms the west but they may be intended more to cool off the union activists and the population generally than to set the stage for an invasion. Similarly, the increasing frequency and strength of warnings from the West could encourage reform leaders to act even more boldly, even though it is not clear what, if anything, the West could do to help if intervention did take place.

Experienced U.S. officials say there is no way to forecast when the breaking point could come because the economic situation in Poland, which compounds worker unhappiness, is so grim, with no improvement in sight.

The Soviets, therefore, face an agonizing decision in which an invasion might forcibly reimpose communist authority but would not solve the underlying economic difficulties, officials here say.

Coming on top of the invasion of Afghanistan last year, a military move into Poland, in the view of U.S. and European specialists, would cost Moscow greatly in the Third World, would end detente in Western Europe and would start the Kremlin off in the worst possible way with a new American administration.

In recent days, Moscow has also ordered that virtually the entire border region between Poland and East Germany be closed to western military observers between Nov. 29 and Dec. 9. Defense officials here said the closure appeared to be linked with a preplanned Soviet air defense exercise similar to previous ones. U.S. military observers are able to do some traveling in East Germany under allied agreements dating back to World War II.

Although Pentagon officials expressed no alarm over the exercise, officials said it was one more example of preparedness moves that have to be carefully watched, because another way for Soviet military intervention to come swiftly and with little warning would be under the guise of a previously scheduled military maneuver along the border.