A thousand people in evening dress turned out for a film on Huey Long last night -- he of the purple shirt, pink tie, white jacket and Charlie McCarthy grin -- to commemorate or celebrate his assassination 50 years ago.

The Frank Nelson Doubleday Lectures, given usually at the National Museum of American History, were moved to the Departmental Auditorium which holds more people and where the sound system makes -- or made -- sufficient noise to wake the dead.

The 90-minute documentary hovered back and forth among a dozen or so people who shared a few insights, then on to the next one, with occasional printed titles to tie things together in an oral history essay.

The Louisiana friends of the senator who were grass-roots plain folk who supported him, were often touching in their tributes to his devotion to the poor, while others like the pundit I.F. Stone and the senior Mrs. Hodding Carter confessed (with slight apologies for feeling this) they were glad when he died. Not that they wanted someone to murder him, but since somebody did -- a physician, Dr. Carl Austin Weiss Jr. of New Orleans -- they were pleased he did not recover.

Huey Long, the closest thing to an American dictator in the nation's history thus far, rose from modest origins in the poorest part of the state to become governor of Louisiana, then U.S. senator, then a serious and possibly powerful candidate for the presidency.

Unlike some demagogues, the senator had a good mind and a fine command of the English language as the film well demonstrated. Direct, colorful, free of barbarisms. One reason the poor may have loved him was they could understand his words, and such images as the poor invited to a feast where the Mellons, Rockefellers and so on snatched away 85 percent of the grub. Such language conveys meaning, in a way that bureaucratic gnawing and yammering inversions do not.

Two central points were made. Perhaps 20 times each. Long was a populist, he wanted the poor to have a greater share of the country's good things and in fact delivered (as his predecessors in Louisiana did not) roads, bridges, night schools, a new university, hospitals, free textbooks and supplies for schoolchildren and so on.

On the other hand he enriched himself and his cronies, he amassed virtually all power in the state to himself, he showed open contempt for what is commonly called ethics in government and faith in constitutional processes.

The film is the work of Ken Burns, an award-winning film maker whose documentary on the Shakers was seen recently on public television here.

C. Vann Woodward, the historian, introduced the film which included comments by historian Arthur Schlesinger, who recalled that some people wondered, seeing Long and also the rise of Hitler and Mussolini in Europe, "could our number have come up?"

The film is one of three of its category that will be shown at the forthcoming New York Film Festival, Burns said, and it seemed ideal for television, since it moves about considerably. It might be thought short on analysis of such points as the ease with which dictatorial power was assumed, the indifference or at least ineffectiveness of a large electorate to prevent it. If Huey Long had no faith in the brains of Louisianians and even less faith in the ability of American citizens to ensure a good society when left to their own devices, without a Fu hrer, the film suggests he had good reason to think as he did.

Burns said Sen. Russell Long (D-La.) who did not attend the showing, had seen the film and respected the intent of its makers but could not endorse it since the film, Burns said, was overwhelmingly negative in its judgments of his father. It may well have struck others the film almost broke its back being "fair" to him, gathering its nectar here and there with equal pleasure and lacking a reasoned viewpoint directed by a clear intellect.

The audience watched it with evident signs of enjoyment for an hour and a half, and unlike several past lectures nobody could be found asleep. A reception followed and the guests rated B (not A, but then not F, either) in the eternally difficult lesson set before them at buffet tables, to grab food and retreat and not commit gridlock for those who are never agile enough or pushy enough to get there first.