Todd Duncan, who was being honored as a distinguished artist and favorite citizen of Washington on his 75th birthday, burst into tears and folded his hands over his face. His broad shoulders shook almost uncontrollably as 45 of his students sang "Unchained Melody" for a movie Duncan made in the 1950s.

"I am not thrilled or awed, but numb," said Duncan a few minutes later, now composed but still teary before the 700 people at the dinner-dance in his honor Saturday night at the Sheraton Park.

"If I faint amd die right now, it's all your fault," said Duncan jokingly, resuming his normal dramatic flourish. He embraced Wayne Dirken, the choirmaster of the Washington Cathedral, who led the Duncan-trained voices in a medley of songs from Duncan's theatrical successes.

At the stroke of midnight, glasses of champagne were raised in Duncan's honor. "He's a star in his prime, a legend in his time," said Patrick Hayes, the managing director of the Washington Performing Arts Society, whose women's committee organized the tribute. Duncan served as the first president of the society. Gladys Duncan, the singer's wife of 43 years, who had wiped his glasses while he wept, was the first to raise her glass.

Duncan's career of Broadway shows, concerts, opera, movies and teaching dates back to the 1920s. He achieved intitial international fame in the orignal 1935 production of "Porgy and Bess." For Saturday night's dancing, however, Peter Duchin selected every variety of American music from Cole Porter to Elvis Presley.

FOR Duncan it was an evening of surprises. He hadn't expected to see Sara Lee, his first voice teacher, Hall Bartlett, the producer of the movie "Unchained," his accompanists, William D. Allen and George Malloy, or Mary Crennan of Columbia Artists. "The '40s were very pathetic for the black artist. But Todd never wavered. He would have to sleep in the railroad station but he was still able to write beautiful letters to the office," said Crennan.

"His symphony is one of love and understanding," said Mayor Walter E. Washington as he declare yesterday Todd Duncan Day. The Washington cultural establishement was represented by Livingston Biddle, chairman of the National Endowment for the Arts, Roger Stevens and Martin Feinstein of the Kennedy Center and ViCurtis Hinton of the D.C. Commission of the Arts. Biddle was accompanied by Jacob Lawrence, the black artist.

One of Duncan's most successful students, Philip Booth, who is now in his third season at the Metropolitan Opera, said the essence of Todd Duncan was probably his humanity. "And that's not only the ability to teach but also the larger guidance. I was once notdoing something right and he said to me 'Young man, you can fool a lot of people, but not the old stinker.'"