Soviet military reservists here were put on alert during the night, apparently in connection with the situation in Poland, well-informed sources said.

Shortly after midnight last night, according to the sources, some reservists in several sections of the city were ordered to show up within 30 minutes at prescribed locations.

The scope of the unexpected alert was unable to be determined, and there was no information from other Soviet cities to suggest a nationwide effort. There was no indications here of any other unusual military activity.

Political observers here believe that the purpose of the alert was to test the ability of the armed forces to mobilize reserves quickly in an emergency.

The sources said that the reservists attended meetings at the prescribed locations, then returned home during early morning hours. It was not known what the men were told by their military commanders or why they were called. No official information on the subject was available since the Soviets regard all such activities as military secrets.

Two American defense attaches are currently traveling in the western regions of the Soviet Union near the border with Poland. Their preliminary reports suggested no unusual military activity in the border areas, according to U.S. sources.

Soviet and Warsaw Pact maneuvers in and around Poland have been a nerve-jangling sideshow to the Polish crisis since the formation of Solidarity in August 1980.

The Soviets began a buildup of troops along Poland's borders late last year as the conflict sharpened about reforms in Poland. In April, U.S. officials expressed strong concern when the so-called Soyuz-81 maneuvers of Warsaw Pact forces around Poland, originally described as small scale, were lengthened by two weeks and expanded in scope at the time of a threatened Solidarity general strike. The strike was eventually cancelled.

During those maneuvers, the Soviets reportedly positioned sophisticated communications facilities and other equipment that military observers said at the time could form the infrastructure for an incursion.

Reservists in the Soviet trans-Carpathian region just north of Poland were called up for intensive training in July, and at summer's end new Warsaw Pact maneuvers involving 100,000 troops were staged, coinciding with Solidarity's first national congress and heightened unrest about food shortages.

Meanwhile, Soviet news dispatches from Warsaw today continued to suggest that the martial-law government of Gen. Wojciech Jaruzelski was in firm control of the country.

A Tass new agency report from Warsaw quoted a member of the Polish Politburo, Stefan Olszowski, as fully endorsing the military takeover, in the first statement indicating that the Polish Communist Party supported the crackdown.

Olszowski, a former foreign minister who is considered a hard-liner in the Politburo, called the military takeover "the only correct decision" in a situation in which counterrevolutionary forces sought to seize power.

"The drastic measures, essential for the salvation of the socialist system in Poland and for the restraint of antisocialist and counterrevolutionary forces, were supported by the party," he was quoted as saying.

The official East German news agency ADN reported that Olszowski said Jaruzelski made the decision on the basis of the decisions of the fourth and sixth central committee meetings and with the authorization of the Politburo, backed by the entire party.

Another Tass dispatch reported that leaders of the regional Solidarity board in Poznan and Slupsk had declared themselves in support of the martial-law government.

Tass quoted Z. Rozwalak, of Poznan, as saying: "As former chairman of the regional board of Solidarity, I am hereby declaring that I am breaking with all forces acting in the trade union organization against the legitimate organs of power. I am against the founding of various parties and organizations opposing Poland's socialist character."

Rozwalak, in a statement said to have been sent to Polish television, was quoted as saying that he "fully agrees" that the Polish Communist Party "should exercise its leading role in our Socialist country. I fully support law and order determined by the martial-law decree."

Tass quoted the chairman of the Solidarity board in Slupsk, W. Zerk, as calling for "ending the activity of Solidarity" in that region in a speech to union members.

The Tass reports stressed that the situation in Poland was returning to normal, although a late dispatch tonight said that "a number of dangerous seats still remain where leaders of Solidarity who have not yet been interned are trying to impose confrontation."

Tass said that Solidarity members tonight "provoked outrages by groups of hooligans in Gdansk." It said the Army and police "dispersed these groups of hooligans."

A Tass commentary tonight also issued a blistering attack on the U.S. Congress, whose resolution on Poland it described as an "outrageous interference in internal affairs of the sovereign Polish republic." The Senate this week passed a resolution deploring martial law in Poland.

"The provocative joint resolution," Tass said, "shows that Washington would like to push events in Poland to confrontation and bloodshed and that the process of normalization, restoration of law and order in Poland, does not suit the U.S. administration."

The agency noted with sarcasm that Congress has yet to pass a similar resolution against the military takeovers in Chile and Turkey.

The Polish situation is expected to dominate the summit meeting later this week of Soviet Bloc leaders scheduled to come here.

Today, the Communist leaders of Hungary, Czechoslovakia and Mongolia arrived in Moscow. Jaruzelski had been scheduled to attend the celebration, but it is not clear now whether he would come in view of the developments in Poland.