"Most people, when they meet the real Muhammad Ali, are shocked," said the real Muhammad Ali, standing on the stage of the Kennedy Center's Terrace Theater, looking as powerful as ever, but sounding softer-spoken, more calm and mature than he is most people's memories.

His well-known public image, said the three-time heavyweight boxing champion, was "the work of my imagination, which sold ticket." He recalled watching wrestling matches, as a boy, noticing how the "bad guys" attracted the public's attention, and saying to himself, "That's for me."

"I played the clown," he said. "People would pay money just to see me get beaten and I'd take the money to the bank. It was an act when I said "I am beautiful" or "I am the greatest'; it was true, of course, but I emphasized it."

Ali was the chief speaker and the recipient of a special award last night at a gala preview of the Kennedy Center's two-week "Imagination Celebration 1979," a festival of the arts for children, now in its third year, which will offer 50 free performances between now and April 14.

The festival brings to the Kennedy Center arts programs from schools around the country. The gala sampled a wide variety of these, including a tribute to Duke Ellington, an original opera, "The Toy Shop," by Seymour Barab, a remarkable dance-in-struction class by Jacques d'Amboise, a dramatized folktale and a very spectacular version of "Sleeping Beauty" done in the traditional style of the Japanese Kabuki theater.

Sepaking after the intermission, Ali said he was surprised to see so many empty seats in the hall, "especially since the tickets are free." (The gala was by invitation only, and some seats were empty because the invited congressmen were kept away in special session.)

"From what we have seen so far," Ali said, "if each seat cost $20, it would be well worth the price." He had flown in from California to attend the gala and had to leave early for his flight back.

In the middle of a long, involved discussion of how "every child, by 12 years old, should know their purpose in life," Ali became fascinated by the translator who was putting all his worlds into sign language.

"Is she keepin' up with me?" he asked, while the translator busily signed his words. "She's movin', ain't she. You all understand what she's doin"?"

"She could be talking about me," he reflected, before getting on with his speech, clearly enjoying the idea.

Also enjoying himself was d' Amboise, who received the Kennedy Center's Excellence Award for his work with children, but said it was "like being rewarded for having fun."