Soviet President Leonid Brezhnev's sudden trip to Prague "emphasizes the seriousness" the Kremlin attaches to resolving the Polish crisis one way or the other, Defense Secretary Caspar W. Weinberger said here tonight.

Brezhnev's mission may be to whip Communist satellites into line as a prelude to using their troops to invade Poland, said Weinberger, giving what he termed one of the "unfortunate hypotheses."

Except for Afghanistan, Weinberger noted, other troops than their own" to suppress an errant like Poland.

As Weinberger spoke with reporters at 6 p.m. London time (noon EST), he assumed from reports out of Warsaw Pact countries that other leaders besides Brezhnev would go to Prague. Weinberger had held out hopes for "some significant opposition" at such a meeting to any harsh measures Brezhnev might insist upon to suppress the independent labor movement in Poland

However, even if Brezhnev proved to be the only visiting help cool down the Polish situation, as Weinberger sees it. "Anything that causes delays" in such drastic action as an armed invasion is "welcome," he said.

Weinberger admitted that neither he nor anyone else in the Reagan administration could tell for sure what the Soviets were doing about Poland. It is only known, he said, that the high tempo of Soviet military activity continues. This could be an attempt to press Polish leaders to crack down on the Solidarity labor movement, to coerce the labor leaders themselves or simply to continue large-scale military maneuvers.

"A lot of this is consistent with a pattern of coercion or intimidation but not intending to go all the way," said Weinberger in examining the Polish crisis as if it were the kind of tangled case he argue as a lawyer in private life.

"It's also consistent with the preparations for a far more serious, direct military intervention. And still to some diminishing extent," Weinberger continued, the stepped-up military activities around Poland are "consistent with expanded and greatly elongated maneuvers. It is very hard to read what they are planning." g

Weinberger and other U.S. officials here demonstrated that they intend to keep warning Moscow of dire consequences if it intervenes in Poland.

President Reagan, from his hospital bed, approved new protest notes to the Soviets over the weekend, warning that an invasion of Poland would doom prospects for a U.S. Soviet summit meeting and arms talks in the foreseeable future. Weinberger also came up with a new complaint tonight, declaring that the Soviets may be violating the Helsinki accords by conducting such massive maneuvers without first notifying other signers of the 1975 agreements.

Weinberger stopped short of saying the flights of Soviet transports in and out of Poland, first announced by the U.S. government, constituted an airlift preparatory to direct military action. But he did say that the resupply effort continues at the same level it was on Friday when the State Department indicated that it thought some kind of intervention was imminent.

Asked if there would be any U.S. military response if the Soviet Union ordered an invasion of Poland, either with its own or satellite troops, Weinberger replied:

"We don't have any so-called military response -- plans that are all automatically ready to go into force -- or anything of that kind of all. I think that when we're talking about response reaction, we're talking about economic, political, diplomatic.

"That involves such things as further summit meeetings or further discussion on limitations of arms." Asked of he would take the precaution of recommending an alert of U.S. troops if the Soviets intervened in Poland, Weinberger stuck to his restrained scenario, declaring: "The alert condition can be raised at any time. It's a very responsive system. But there's no suggestion it would be necessary to do that."

As for extricating U.S. citizens in Poland if there is military action there, Weinberger said that those people "will always be a matter of obvious concern" to the U.S. government, but that no special preparations have been made at this point.

Weinberger leaves Britain Monday for Bonn, where he is to attend a meeting of the Nuclear Planning Group, made up of defense ministers from NATO nations.