Twelve-year-old Keith Robinson wants to be a boxer. And all week long, he waited for Saturday to come.

He talked about it with his friend Derrick, who owns a real pair of boxing gloves and sometimes boxes with him on the streets of their Southeast Washington neighborhood. It was the talk of the neighborhood recreation center, and his teachers talked about it at school.

Yesterday morning, it took Keith two buses and one hour and 10 minutes to get from his home to the Washington Monument grounds. But the sleepy bus ride ended in a dream-come-true -- Keith got to stand in a boxing ring with his all-time hero, Muhammad Ali, shake his hand and even pretend at one point to be brushing the boxer's teeth.

Ali was in town yesterday to launch a national campaign against tooth decay and to work his sort of Pied Piper magic on a crowd of about 2,500 youngsters and adults who huddled together in the bone-chilling wind to see Ali battle it out with his latest opponent, the "pride of Cavityville," Mr. Tooth Decay.

Prancing about in a blaze of red silk, jabbing his hand in the air and dazzling his awe-struck admirers with his familiar brand of verbal pyrotechnics, Ali boomed, "Here he come, here come Mr. Tooth Decay," as his opponent, in a white cape and mask, strode into the ring.

"I want you all to hate tooth decay," he shouted, jumping up and down as the crowd cheered "Al-lee, Al-lee."

In the ring with him was Mayor Marion Barry, D.C. school superintendent Vincent E. Reed, Montgomery county school superintendent Edward Andrews -- all referees -- and D.C. Del. Walter Fauntroy, who wore a dark blue velvet-trimmed tuxedo.

The officials tried to take advantage of the event to do some politicking, walking close behind Ali, and shaking the hands that stretched out to touch the erstwhile champion.

"Who's that?" one woman in the crowd asked another who had just shaken the hand of a tall man in a gray vested suit, following Ali. "Oh, that's Marion Barry," the woman replied.

It was clear whom the people had come to see. Michael Webb brought his wife, son, and two daughters to see Ali, who Webb says is "not just an athlete, but a political and social figure (who's) made his place in history.

"I followed him from the time I was a teen-ager," Webb said, "from the time he was in the Olympics to the time he got interested in Malcolm X," the black activist.

He would take his nine-year-old son to see Ali at the D.C. Armory, and when Ali lost the heavyweight championship to Leon Spinks, Webb said his eight-year-old daughter, another Ali fan cried.

Ali seemed to make the biggest hit with the hundreds of young street boxers like Keith Robinson and his friend Michael James. At 15, Michael has had some reading problems in the past. But he's bought three books on Ali in the past two years, and reads and re-reads the sections on boxing.

At times it looked as though Mr. Tooth Decay, played by boxer Chuck Wepner, who fought Ali in 1975, had forced Ali to the floor. But then, the crowd cheered even louder for Ali, and boys like Keith ran up to their hero with toothbrushes, and pretended to brush his teeth. That would make Mr. Tooth Decay shrink back as if attacked.

"Want to see me get him?" Ali would shout, leaning over the red, white and blue ropes. "Yeah," the children would shriek.

"Boo for tooth decay! Knock 'im out, Ali," shouted 10-year-old Anthony Gorden, who was pressed against the front of the fence around the ring.

"Listen l'il children, I'm gonna knock him out in the sixth round," Ali told the crowd.

He did -- and he celebrated his victory by throwing the children toothbrushes and warning, "Don't you think because I knocked out tooth decay it's all over. He's still out there . . . You must brush your teeth every day."

Yesterday's match was sponsored free of charge, by A&P supermarkets, which will be distributing an album on tooth decay called "May the Brush Be With You." It features the voices of Ali, Frank Sinatra, Billie Jean King, Lily Tomlin and President Carter.Most of the proceeds will go to charity -- and the rest to Ali.