U.S. Secretary of State Edmund Muskie arrived here today for a NATO foreign ministers' meeting and began to seek agreement on specific allied economic and political moves in the event of a Soviet invasion of Poland.

Muskie, armed with a list of possible measures to be taken if the Soviets intervene, said he hoped he and his NATO counterparts could "get some constructive work done in dealing with the contingencies that lie ahead." He met privately during the day with several allied foreign ministers.

As Muskie prepared for the two-day conference starting Thursday, Defense Secretary Harold Brown said in a speech at the conclusion of a NATO defense ministers' meeting today that a Soviet invasion of Poland would prompt a further buildup of Western military forces.

Concluding the two-day defense ministers' meeting, Brown declared, "The seriousness of consequences of a Soviet intervention in Poland should be understood by the Soviets before they take any such action."

He added, "Those consequences would range over a broad spectrum of political, economic and diplomatic activities, and there's no doubt in my mind that the West would also have to react by further building up its military capability." He said such a buildup would exceed that already planned by the Alliance.

[Shortly after Muskie's arrival here, United Press International quoted officials aboard the secretary's plane as saying that the United States would seek an allied commitment to trigger sanctions automatically against the Soviets and their allies of Poland were invaded.]

Brown's statement was met initially with indications of support from West European defense leaders, some of whose countries have had difficulty meeting existing military commitments to the North Atlantic Treaty Organization (NATO). But the final communique issued today by NATO's defense planning committee avoided spelling out specific consequences in the event of a Soviet invasion.

"Ministers voiced deep concern with the situation relating to Poland and agreed that any military intervention would pose a serious threat to security and stability with profound implications for all facts of the East-West relationship," the communique said in its single reference to Poland.

The NATO defense chiefs were said to have approved only routine precautionary measures of a defensive nature in connection with events in Poland. These included a request yesterday for four sophisticated radar surveillence planes to be flown from the United States for exercises in West Germany by Thursday.

While Western officials have firmly warned the Soviets against moving into Poland, West European governments appear deeply reluctant to list likely repercussions before the fact. The laying of some of the groundwork for cooperative sanctions against Moscow if an invasion occurs is expected to be a central focus in consultations here Thursday and Friday among NATO foreign ministers.

UPI quoted Muskie as telling reporters on the plane to Brussels that the use of force by NATO in response to a Soivet invasion of Poland could not be ruled out. "Poland is 200 miles from NATO territory, NATO is created to defend the territory, and there is the possibility of the use of force," Muskie was quoted as saying.

[However, State Department spokesman John Trattner latter denied the statement, saying that Muskie had left unanswered seveal questions about the possible use of force.]

In Washington, administration sources said Muskie took with him to Brussels several U.S. ideas for joint action with allies to counter a possible Soviet invasion. The sources said the U.S. suggestions include:

A political demonstration at the Madrid review conference on the Helsinki accords, on the grounds that a Soviet military intervention would be a massive violation of the 1975 agreements. The demonstration might take the form of an allied walkout or termination of the Madrid meeting. Muskie said last week that a Soviet invasion of Poland "could well destroy" the Helsinki accords.

"More of the same" economic sanctions that the United States and other Western countries adopted against the Soviet Union early this year following the Soviet invasion of Afghanistan. The economic counter-measures being suggested by the United States include both trade and investment. Among the projects that the United States would like to stop, according to the sources, is a multibillion-dollar gas pipeline from Soviet Siberia to Western Europe. However, the Carter administration recently authorized the Caterpillar Tractor Co. to bid on the supply of pipe-laying equipment for the project.

Cutbacks in political and cultural contacts between the Soviet Union and the West. Because of earlier U.S. action as a result of the Afghanistan invasion, only minimum U.S.-Soviet activities remain. Among the items under discussion in the administration, sources said, is a reduction in the size of the U.S. Embassy in Moscow and the Soviet Embassy in Washington.

Application of economic and political sanctions to those eastern European countries whose military forces participate in any armed intervention in Poland. In the case of Afghanistan, Washington drew a clear line of difference between policy toward the Soviets and toward their East Bloc allies. This time sanctions would be applied to Eastern European countries, depending on the nature of their participation, sources said.

U.S. sources said the State Department has been engaged in informal conversations with the European allies for several months on the Western response to possible Soviet military action in Poland.