"Invasion," a British-made docu-drama about the Soviet invasion of Czechoslovakia in 1968 (Channel 7 tonight at 9:30) is a powerful and moving piece of work, spare in its characterizations yet eloquent in its message.

With Soviet bloc troops now poised at Poland's borders on the chance that popular demands for political and social reforms there may get out of hand, the film is especially relevant. The same Leonid Brezhnev, who as Soviet Communist Party boss bullied Czechoslovak leader Alexander Dubcek 12 years ago, is still in charge at the Kremlin, ready to apply similar measures to the Poles.

Poland today and Czechoslovakia in 1968 are different places, and direct comparisons at this point are premature. Nonetheless, as a record of what happened to the Czechoslovaks and an indicator of what may await the Poles, "Invasion" is a graphic testimonial.

ABC is presenting the 90-minute film as a news special and following it at 11:30 with a special edition of Ted Koppel's "Nightline" about Eastern Europe. In fact, "Invasion" is more a dramatization than it is news and should not be confused with word-for-word recreations of past events such as those tried, for instance, with Watergate tapes. Some of the dialogue has clearly been souped up for the sake of impact; some of the action is obviously telescoped in time.

Still, as a portrayal of events in the week beginning late on the night of Aug. 19 when the invasion began and ending Aug. 27 with the return of Czechoslovak leaders from a showdown at the Kremlin, the program is as convincing as a work of this kind can be. The overall tension is real. The helplessness of the Czechoslovaks is poignant. The smug brutality of the Soviet leaders is infuriating.

The scene shifts from the party headquarters in Prague, where through the window the sound of advancing Soviet forces can be heard, to the Kremlin headquarters of the Moscow leadership. The Czechoslovaks are taken into "protective custody," later arrested and then propped up to act as negotiators for their country's fate with the Soviet captors. Virtually all the action is indoors.

The main source for the film is Zdenek Mlynar, who was secretary of the Communist Party presidium under Dubcek and now lives in western Europe. He recorded more than 26 hours of recollections for the staff of Britain's Granada Television which, ABC says, then spent two years verifying as much of the account as possible. Understandably, the story unfolds from Mlynar's perspective, which may distort somewhat the role of others.

Dubcek, who is remembered now as the motivating force behind the transformation of Czechoslovakia's hardline Communist Party to "socialism with a human face," seems almost pathetically unhinged by the perfidy of his Soviet allies. After he and his closest colleagues were bundled off to Moscow by Soviet occupiers, Dubcek collapsed and was bedridden during the crucial days of bargaining with Brezhnev and the others. The impression left -- probably correct -- is that Dubcek was simply not strong enough to face down the invaders.

The rest of the Czechoslovaks -- Oldrich Cernik, the prime minister; Ludovik Svoboda, the president; Gustav Husak, who was eventually the Soviets' choice to replace Dubcek -- are believably acted in ways that conform to the parts they are known to have played.

The Soviets are perhaps best of all. Brezhnev and the late Alexander Kosygin are superbly made up and their dialogue, based on all we know of what was said at the time, is faithful. The climactic confrontation in the Kremlin at the end of which Brezhnev stalks out and leaves Dubcek gasping, conveys a starker sense of the Kremlin's inner sanctums than anything I've ever seen or read.

Now, many viewers have doubtless forgotten some of the details of the tragic end to the Prague Spring (as Dubcek's reform movement was called) and the fast-paced action may seem confusing at times. But the emotion comes through even if the precise chronology gets fuzzy. It is the sort of gripping drama that one feels in watching a jury's life-and-death deliberations: a great deal of talk that never gets boring.

The film, which was directed by Leslie Woodhead and written by David Boulton of Granada, ends with an update on what has happened to the principal Czechoslovaks. Most have fared badly. And that, alas, is true of their country as well. The vitality and hope that comes through in newsreels at the program's start is gone from Czechoslovakia, the most melancholy of Soviet-bloc nations. "Invasion" goes right to the roots of that sorrow.