A small explosion and fire damaged the downtown office of Aeroflot, the Soviet national airline, early this morning only hours before the top Polish leadership met in Moscow with Soviet leader Leonid Brezhnev over this country's growing political and economic confrontation.

No one was injured, the cause of the blast is unknown, and Warsaw police would rather not discuss the incident which brought to the surface -- in a way few other incidents could -- the razor's edge on which the Poles now are walking.

A provacation against Moscow? A portent of things to come? It's anyone's guess given the existing uncertain sense of events here.

The Polish stage is set once again for a tense drama Friday, pitting communist authorities holding fast to ideology and power against an invigorated, newly established mass labor movement.

Poised for a strike if the talks prove unsatisfactory, representatives of the country's independent trade union organization Solidarity sit down with Premier Jozef Pinkowski hoping to win cooperative government action on several urgent grievances.

The situation would be strained enough if it were just an internal Polish matter. But the rest of Soviet-led Eastern Europe -- which has been taking longer to get used to the idea of free unions in neighboring Poland than have the Polish authorities -- has taken an active interest in seeing that Warsaw communist rule remains firm.

Solidarity leaders said today they were not worried by the sudden trip to Moscow by Polish Communist Party chief Stanislaw Kania and Premier Pinkowski, at which the upcoming talks with the union were thought to have been discussed. The Moscow visit was viewed by labor organizers here as predictable and normal consultations rather than as an alarming signal of a new get-tough Soviet attitude.

But just to impress upon authorities how sizable their own union troops are -- estimated variously to number between 6 million and 10 million -- Solidarity will be sending a delegation of about 60 people. Included will be the national coordinating committee made up of representatives from the union's 40-odd regional branches. In addition, the full leadership of the Gdansk branch, the leading militant group, will be at the talks, along with the so-called group of intellectual experts.

In contrast to the worker-government confrontations of the summer in strike-bound Baltic shipyards or outside Silesian mines, Friday's session will take place not in worker territory but in the official setting of Warsaw's Council of Ministers headquarters.

While Solidarity leaders would have preferred to meet on their own turf in Gdansk, the establishment surroundings do reflect the legitimacy and importance the young union has gained in Polish society since the first independent worker groups began to form two months ago.

At stake for the Polish government in the talks is its delicate balance between, on the one hand, keeping social peace at home by compromising with an angry and determined Solidarity and, on the other, keeping the Soviet faith.

At stake for Solidarity is, externally, its recently granted independence from the government and Polish Communist Party, and internally, its sense of unity.

By demanding too much and forcing a strike, the union could bring down the roof on its own experiment. By settling for too little, it could engender a split in its own ranks between moderate and radical groups.

Weighing the possible gains and losses of a strike has become more complicated for Polish workers in recent weeks. Before, they had nothing to lose. Now, they have a union and other new freedoms guaranteed by the summer agreements. In addition, further work stoppages at this time would risk economic catastrophe in Poland.

Because of the worsening shortages of goods since the summer, many Poles do not want another strike.But should Solidarity opt for one, it is sure to get widespread support. The reason is that Solidarity is viewed as a critical new chance which workers are likely to support in causes which might go against their better judgment.

"If we ask, 'Should there be a strike,' the country is indecisive. If we announce a strike, the entire country will be with us," said Andrzej Gwiazda, a leader of the Gdansk union, in an interview this week.

While increasingly conscious of their growing responsibility to both the dreams and the economy of the nation, the union federation itself tends still to be consumed by the individual emotions and ambitions of its organizers.

It does not have formally elected officers yet; its governing structure often appears unwieldy; and its commission meetings frequently turn heated and fall into chaos.

The arguments and divisions among members are seen as a natural function of the lack of democratic experience among workers and the strong personalities of some leaders. But one question being asked is whether Solidarity can get a grip on itself in time to keep its grip on the country.

The Warsaw regime has taken every opportunity since signing the Aug. 31 agreement which guaranteed the existence of independent trade unions to affirm that guarantee. But tomorrow's discussion relates to the one theme on which the Polish government can brook no compromise: its communist principles.

These principles -- which include the leading role of the party in the state, social ownership of the means of production and the inviolability of Poland's international alliances -- were recognized by workers in the Gdansk agreement. At issue is whether Solidarity can agree to write them into its charter.

Union leaders say such political principles have no place in the charter of their nonpolitical organization. Government officials answer that by refusing to include them, Solidarity itself commits a political act.

The Warsaw district court forced the principles in as part of its decision last week to go ahead and register Solidarity -- a tour de force branded as illegal by the union which has announced it will appeal the insertion and other changes dictated by the court.

Going on the assumption that the lower court did not act on its own -- and neither will the Supreme Court in hearing the appeal -- union leaders want assurances from the government that Polish authorities will swing their influence behind Solidarity's cause. Depending on the response, the union has threatened it will prepare selective strikes by Nov. 12.

Four other issues are on Solidarity's agenda: more access to the media, which includes provision for a new trade union paper; further wage increases for groups not adequately covered under the summer agreements; registration of the trade union of peasants, whose statute was rejected on grounds that farmers, most of whom own their own land, do not qualify as workers and so do not come under the Gdansk accord; drafting of real economic reforms to get rid of chronic shortages of goods.