"Apocalypse Now," one of the most ballyhooed movies of the decade, got only a polite response at the Cannes Film Festival on Saturday.

The 20 rows of leading film critics could muster only lukewarm applause for the yet unfinished $30-million Vietnam war film, but the French section of the audience reacted enthusiastically, Jack Valenti, head of the Motion Picture Export Association of America, declared it brilliant.

Following the showing, director Francis Ford Coppola attacked the American press as the most decadent in the world, and complained that journalists had broken their promises not to write about it before its official release. Discussing the cost of the film, he asked, "Why is it that I, the first one to make a film about Vietnam, a film about morality, am so criticized when you can spend that much about a gorilla or a little jerk who flies around in the sky?"

Coppola dismissed the Oscar-winning "The Deer Hunter," also about the Vietnam war, as "politically naive," although he said he admired the acting.

"Apocalypse Now," though listed on the program as a "work in progress," is nevertheless in competition at the festival. Last week, Coppola showed the same unfinished version, trimmed to two hours and 20 minutes, to the movie community inLos Angeles and President Carter and his guests in the White House. In Los Angeles, viewers were asked to complete a questionnaire in which Copola asked them "to help me finish the film."

In Cannes, the consensus was that the first half is a model of film expertise, which the latter stages disintegrate into banality and worse. Howeverr, the acting, editing and photography were generally acknowledged to be outstanding.

The film, a surreal vision of the war in Vietnam loosely based on Joseph Conrad's "Heart of Darness," centers on U.S. patrol's hunt for a demented American colonel, played by Marion Brando, who has established his own empire in Cambodia. The patrol, led by a captain played by Martin Sheen, has been ordered to find the mad American and execute him.

Recalling Conrad, the captain and his patrol boat make their way up a jungle river, where they encounter more and more horrifying events. At one point, a surfing-mad colonel forces his soldiers to ride the waves during an aerial attack. At another, the gyrations of the chorus girls during a USO show make the military audience wild with lust, and the patrol is forced to make a quick get-away by helipcopter. All along, innocent Vietnamese are being killed.

Brando plays the demented colonel largely as a horror-movie freak, a bald-headed, cold-blooded grotesque who recites poetry and membles about making horror an ally. He is aware that he must die - for with his demise his people can return to their rice fields - and the captain beats him to death while the natives are slaughtering cattle for a traditional feast.

"I thought the river journey is like a journey through life," Coppola told the Cannes audience. "There is only one place the journey can take you, which is inside yourself. Vietnam was one of the great elements of our history. It was about the basic questions of morality and that is why I chose to begin this project.

"I began to realize that rather than a film about Vietnam or about war, I was making a film about the precarious postion we are all in - that we must choose between good and evil every moment of our lives. In war, the concept of morality is twisted inside out more than ever. The film is about moral ambiguity, about hypocrisy, about saying one thing and doing another."

"The way we made it is the way the Americans were in Vietnam - in the jungle with too much money and too much equipment," he explained.

Coppola, who had difficulty financing the film, said: "I invested all my money and Own the film. I think I'll get it back."

He said he drew up a list of 200 themes he wanted to treat. "For example, the use of drugs, black soldiers at the front line, American officers living in affluence playing God and eating fine food, and the fact that many soldiers were naive 17-and 18-year olds."

The film has been edited from miles of footage, much of it shot in the Philiphines. Though it was reported that Coppola had devised a violent denunciation of U.S. military action in Vietnam, the movie's current form contains no overt propaganda. It is anti-war, not anti-American. And Coppola has pictured in the opening and building sequences how the shadow of war reigns over all, from Saigon to the borders of Brando's mysterious kingdom. Once we enter there we are lost in psychedelic anxiety and direction and script lose their firm control. CAPTION: Picture, "Apocalypse Now" director Francis Ford Coppola after Cannes screening, by AP