There was a time -- not so long ago -- when authorities here would use bulldozers and high pressure water hoses to break up exhibits of what is known as "unofficial" Soviet art.

Or, if circumstances did not permit the use of such heavy equipment, a lot of plainclothesmen would descend on the scene, disperse the crowd of would-be viewers and shut down exhibits before they could even open.

For some strange reason, this always took place about half an hour before the scheduled opening, as if authorities deliberately sought to raise hopes before shattering them. Canvases were confiscated or damaged. Indignant art lovers, and sometimes Western journalists, would rush up. There was a sense of drama in the air.

This drama was missing last week when about 35 veterans of various abortive exhibitions opened a collective show called Russian Themes '80. The show was at an official gallery and was opened with a speech. Crowds were there to view works that ranged from abstract to religious. And the only piece of heavy equipment was a Moscow television truck, parked outside the gallery to record the occasion.

As art, it may have been mediocre but as political event it was noteworthy. The show symbolized a process of accommodation over the past several years, an effort to co-opt unofficial artists without loosening the traditional strict internal controls.

Until recently, the Soviet Union, once having excluded an artist from the official artists union, acted as if he didn't exist. An unofficial artist could not show his works -- they were automatically ruled unacceptable -- presumably because they were likely to spread some social or ideological malady.

One example of the changing artistic climate is Vadim Sidur, who, despite official disapproval, has gained recognition among the country's cultural elite as one of the finest sculptors and graphic artists in Moscow.

Sidur, 56, has had several exhibitions in the West, but not a single show in his own country. On Aug. 28, an exhibition of his work opened in West Berlin. A glossy catalogue somewhat compensated for his disappointment over not being allowed to be present at the opening.

Son of a Jewish father and Russian mother, Sidur was a Red Army machine-gunner during World War II. He is not a political dissident, but neither is he a man who wants to follow the pack.

During the past two years, Sidur's works have begun showing up on Moscow's streets. Last week, one of his abstract sculptures -- 10 feet high and 33 feet wide -- was unveiled in front of the Institute of Geochemistry. Another even larger sculpture -- 50 feet wide and 14 feet high -- is located in front of the Morphological Institute of the Academy of Medical Sciences.

The seeming acceptance of this artistic outcast reflects an increasing complexity in Soviet society. Powerful scientific leaders whose institutions have money and political clout can now defy the cultural watchdogs who sit on various committees.

A few abstract sculptures and exhibits, however insignificant they may seem, represent a certain change of mood in this country, which is almost heroically resistant to change. At the very least, there are visible pressure groups that, while not challenging the crucial aspects of the authoritarian society, are becoming more assertive on peripheral issues such as the visual arts.