Soviet President Leonid Brezhnev flew from his Black Sea holiday retreat to Moscow today amid signs of increased nervousness here about the political crisis in Poland.

Although there was no announcement that Brezhnev had cut his vacation short to return here, diplomats believe that his presence in the capital reflects that seriousness of Moscow's concern about events in Poland. c

Tonight, the government news agency Tass distributed in Russian its first substantive reports on the crisis, in which it acknowledged that Poland's economic situation was "grave," and outlined concessions offered to striking workers by Polish leader Edward Gierek last night. Such distribution of unsettling news by the Tass Russian-language service in highly unusual.

Meanwhile, the official East German news agency ADN reported that a Bulgarian artillery unit arrived today in northern East Germany for a Warsaw Pact military excercise. The maneuver site is only 60 miles west of northern Poland.

The fact that East Germany announced the arrival of the artillery unit is seen as a further indication of the grave concern with which the Warsaw Pact countries regard the events in Poland.

There were reports tonight that a Czechoslovak tank unit had also arrived in East Germany in preparation for the maneuvers, scheduled for early September.

A total of 40,000 troops from all Soviet Bloc countries except Romania are scheduled to take part in the war games, some of which will be held in areas near Poland's Baltic coast, the center of unrest.

The Soviet press during the past two days has been stressing the need for unity among the Warsaw Pact nations, and a Tass commentary tonight underscored that only as a socialist country could Poland "advance forward in all spheres, including the development of its economy."

The Soviets are believed to be prepared to go to great lengths to avoid military intervention. The hope here clearly is that the crisis will be resolved by the Polish leadership without violence.

But the apparent impasse, that followed the dramatic reshuffle of the Polish Communist Party and government leadership last night is seen by diplomats as having weakened Gierek's position.

In this view, the time may be running for the Polish communist leader if the striking workers reject his proposals despite government changes and concessions announced last night.

According to Western diplomats, the Russians have two armored divisions stationed in Poland, with a combined strength of 20,000 men. These forces are said to be keeping an extremely low profile, as they have in the past. The Soviet troops in Poland have been confined to their bases, and only officers -- in civilian clothes -- are allowed to leave the compounds.

In addition to these forces, the Russians have substantial troop concentrations in the western parts of the Soviet Union that could easily quell a major outbreak of violence in Poland. s

But Western sources here say that the Russians would be extremely loath to use military force in Poland. One reason is Moscow's desire to avoid any problems with East-West relations -- already strained as a result of its invasion of Afghanistan last December.

But Poland presents a specific problem. Unlike Czechoslovakia, which the Warsaw Pact troops invaded in 1968 to end Alexander Dubcek's liberalization, the Polish unrest involves tens of thousands of striking industrial workers who tasted power when they ousted Gierek's predecessor in 1970. Those workers have already forced Gierek to offer major concessions.

The Polish workers may well resist any repressive force. Moreover, if Gierek decided to use force against them, the Polish Army might refuse to fight its own people and that, according to diplomats, would make Soviet intervention inevitable.

Western observers say that for these reasons, neither Gierek nor the Russians are likely to pursue military solutions.

But Gierek's concessions on the formation of free trade unions and liberalization of the press have placed his government on a slippery slope. While viewed as a tactical move, they nevertheless appear to lead to political pluralism, and therefore are not acceptable to Moscow and some other East European countries.

Tonight, the Tass commentary assailed unspecified Western groups for trying to use the Polish unrest to "blacken and discredit socialism." It said the Western press was trying to "depict stoppages at a number of Polish enterprises as a renunciation of socialism."

Moscow television singled out West Germany for criticism, saying that one candidate for West German chancellor -- Franz Josef Strauss -- had made speeches "just stopping short of demands annexation of former German territories" that are now part of Poland.

The commentators, however, made no mention of specific workers demands and omitted references that are regarded as possibly unsettling for Soviet workers.

Although restrained in tone, the commentaries underscored Moscow's view by quoting with approval phrases from Gierek's speech that asserted that no demands by workers would be considered if they are a "direct attack" on Poland's socialist system.