The Cannes Film Festival, the mecca of European filmmakers who once scorned Hollywood as the home of the rich moguls who make low-grade movies, awarded most of its top honors to American films yesterday.

"Apocalypse Now," Francis Ford Coppola's unfinished $30-million Vietnam War epic, shared the Golden Palm award for best film with West German director Volker Schlondorff's "The Tin Drum."

Jack Lemmon was named best actor for his portrayal of a nuclear reactor technician in "The China Syndrome," a movie about an accident in an atomic power plant. "I'm happy to be honored for my first non-comic role," Lemmon, 56, told reporters.

Sally Field picked up the best-actress award for her title role in "Norma Rae," a chronicle of attempts to unionize workers in an Alabama textile factory.

Another American, Terrence Malick, 36, was named best director for "Days of Heaven," a harrowing, exquisitely photographed story about love and greed set in the wheatfields of Texas just before World War I.

An international jury of cinema venerables, ranging from Indian director Satyajit Ray to English actress Susannah York, honored "Apocalypse Now" and "The China Syndrome" even though they were panned by many European critics. "Apocalypse Now" is still unreleased, with an "in progress" version shown at the annual festival. Critics reacted with a mixture of applause and catcalls when it was given the award.

The United States dominated the festival to an even greater extent than the awards indicate. "Hair," "Manhattan" and John Huston's "Wise Blood" were shown and widely discussed. Not long ago, the major American studios shied away from the competition, fearing the barbs of hostile critics: This time they appeared in force and conquered.

It was only the eighth time since 1946 that Cannes judges have chosen an American film for the top award. And no picture had a great impact than "Apocalypse Now," which simultaneously overwhelmed and bewildered the audience. Coppola gave a memorable press conference, describing his picture as "the world's first $30 million surrealistic movie . . . a theatrical-film-myth dealing with the theme of moral ambiguity. It was made the way the vietnam war was fought; in the jungle," he said. "There were too many of us, too much money, too much equipment . . . we went insane."

The co-winner, Volker Schloendorff's "The Tin Drum," was adapted from the Gunter Grass novel about Nazi Germany. It begins at the turn of the century, in a landscape so bleak and muddy that it has to be allegorical. A fugitive hides from pursuing soldiers under the skirts of a peasant woman. Moments later, as the soldiers depart, the woman's ecstatic moans indicate that the fugitive has not wasted his opportunity. The scene sets the tone for the film: high seriousness undermined by black humor.

On his third birthday, the peasant woman's grandson, Oscar, is given a tin drum. He refuses ever to be parted from it, stops growing, and, as a manic dwarf, embarks on a sinister career. His eyes are mesmerizing, his shriek shatters glass. He disrupts a Nazi rally by insistently drumming to a different beat, dons an SS uniform and joins a troupe of circus dwarfs who entertain front-line soldiers, seduces a teen-age girl who cannot believe he is a man. Grass refused many offers to film his book: Schleondorff succeeds admirably in capturing its dark insights into the German spirit, its mordant humor and the dominating personality of Oscar.

The 11-member international jury, chaired by French novelist Francoise Sagan, heard catcalls of "demagogues" and "fence-sitters" as the awards were read out.

As Sagan Explained afterward, the choice was difficult but the jury was satisfied. She added they "found there were not at all that many great performances" to choose from.

The decision to split the top award eased tension at the festival and avoided the uproar that would have resulted from giving the top prize to Coppola's "work in progress."

Twenty-one films were entered in the 32nd Cannes festival, including ones from Israel and Australia.

A common theme of many films has been the struggle for personal freedom - often against great odds. Nowhere was this better demonstrated than in the Australian entry, "My Brilliant Career," one of several remarkable festival films by women directors. Gillian Armstrong's first feature is based on a novel written in 1901 by a 16-year-old girl. It's a about a proud, rebellious teen-ager, who refuses offers of marriage - though they offer her the only hope of security - in order to make a career as a writer. Her personal drama is framed by a stunning evocation of frontier society in the 1890s: polarized between squalor and gentility.

Fellini's "Orchestra Rehearsal," made a year ago for Italian television, but long withheld from exhibition, is another film that could be read as historical allegory, though Fellini claims it is "An ethical, not a political, fable."

Musicians gather in an austerely beautiful medieval chapel - to rehearse and be interviewed for television. Each brags that his instrument is the most essential or versatile. The rehearsal begins, but goes badly: the conductor becomes hysterical, the musicians rebellious. The union leader proposes a break.

When the conductor returns, he is confronted by an angry mob. Graffiti cover the walls. There are threats of violence. Suddenly, a huge wrecker's ball demolishes one wall of the chapel. The musicians, covered in debris, are stilled. The conductor resumes the rehearsal, this time with greater authority, barking his commands in German. . . .

The political implications are obvious, but the film can be enjoyed on a different level. It pulsates with Fellini's love of music and the cranky, talented people who create it. It is dedicated to the late Nino Rota, who scored nearly all of Fellini's films.

This year's British entry was "The Europeans," directed by the American James Ivoy, produced by an Italian, Ismail Merchant, part-financed in Germany, and shot in New England. Despite its hybrid origins, "The Europeans" does have a predominantly English cast. In the film two Europeans pay a surprise visit to their Boston Brahmin cousins, and are welcomed with curiosity and apprehension. The visitors - a baroness looking for a new husband (Lee Remick) and a young Bohemian with an eye for women (Tim Woodward) - are installed in a house on the family estate. An elegant, ritualized dance of courtship ensues under father's disapproving gaze.

Like "The Duellists," this is a film of great visual beauty that captures the feeling of period without descending to the merely pictorial. The script is by the Indian writer, Ruth Prawer Jhabvala, who has adapted the Henry James novel with extrarodinary finesse, preserving the atmosphere, sharpening the characterizations.

Finally, the critics were introduced to an exciting new Japanese director, Hiroko Yokoyama. "Jun" is a film of effervescence and striking visual style: a portrait of young people in Tokyo, and the study of an obsession. A polite young man with a steady job and charming girlfriend is irresistibly driven to caress women in the crowded subway. As he pursues his obsession the victim merely shudders and the other men observe with polite curiosity. Other Japanese directors have dealt with obsessions, but seldom have they been treated so cooly, and the spectator made to feel a participant rather than a voyeur. CAPTION: Picture 1, Cannes Best Actor, Jack Lemmon, and Actress, Sally Field, by AP; Picture 2, "The China Syndrome" star Jack Lemmon signals victory, by UPI