WASHINGTON (AP) _ Some visitors pondering the new bust of Martin Luther King Jr. in the Capitol Rotunda seem surprised, annoyed or perplexed by the pensive image of the slain civil rights leader gazing down from its black marble pedestal.

The sculpture, unveiled Jan. 16 by his widow, Coretta Scott King, has stirred misgivings among some of King's admirers who remember him as the lively and bold prophet of non-violent revolution who preached racial justice in Birmingham, Selma and Memphis.

They are proud that King has taken ''his rightful place among the heroes of this nation,'' as Sen. Charles McC. Mathias, R-Md., said, but they express disappointment that King is portrayed with what they regard as uncharacteristic meekness and humility.

The criticism prompts a spirited defense of the rendering by artist John Wilson of Boston and by the chairman of the panel that selected Wilson for the $50,000 government commission to execute the first sculpture of a black American to be placed in the Capitol.

Spokesmen for Mrs. King said last week that she was vacationing and not available for comment. But her son, Dexter, 24, said the bust was ''a very good likeness'' of his father and ''represents a kind of youthful aspect that is very important.''

The bronze sculpture, 8 feet 6 inches high atop its pedestal, depicts a brooding young King, head slightly turned and downcast, his eyes half-closed as if in reverie.

''It makes him look sort of subservient, without much life or character, with his head resting on a rather limp body. I would like to see him stand a little straighter and a little more spirited,'' said Gloria Buck of Newark, N.J., during a King holiday visit to the Rotunda.

Her husband, Clement Price, a member of the New Jersey State Council on the Arts, disagreed. ''I like it very much. It's an imaginative likeness. The demeanor captures the spirit of the Afro-American people, which is humility, but also King's own sense of humility and grace,'' he said.

''This is much more of a reflective type of King, but I don't think he was ever that quiet,'' said Dr. Thomas Gay of Largo, Md. ''I think that's why people are disappointed in this bust.''

''King was a really fiery kind of guy who could express himself well and get people to follow him. This doesn't capture how dynamic a person he really was,'' said Gay, who was a college student in Atlanta in the early 1960s when he first met King.

''It's too subdued,'' agreed Gay's wife, Betty. ''It doesn't look very much like him.''

''I always picture him as speaking, a man of action exhorting his followers,'' said Marie Cunningham Brown, a local resident who was stirred by King's oratory at the 1963 civil rights march on Washington. ''To see him in a solemn pose is a little disconcerting.''

An aide to Rep. John Conyers, D-Mich., a member of the Congressional Black Caucus, said ''quite a few people had called to express disappointment'' over the King sculpture. Conyers, he said, had not yet seen the bust.

Rep. Parren Mitchell, D-Md., a King ally during the civil rights struggles of the 1960s, conceded that ''some of my friends say the bust will have to grow on them.'' Personally, Mitchell thinks it's a ''fantastic'' work, and Rep. Walter Fauntroy, D-D.C., applauded ''the pensive nature of the portrayal.''

In a telephone interview from Boston, sculptor Wilson said he's not surprised that his rendering is disliked by some people who have their own, personal images of King.

''People read into it what they bring to it,'' he said.

''But humility had absolutely nothing to do with my piece. King's head is tilted forward - not bowed - so that someone standing below will have a kind of eyeball-to-eyeball confrontation with him,'' Wilson said.

''I wanted to show that kind of brooding, contemplative, inner-directed person that's the essence of the man, his eloquence in communicating his ideas and his compassion for people.

''The power of the man, which I suggest in his bull-like neck, lay in his profound philosophy, his spiritual strength and the power of his ideas, which are out there now, changing the world. The greatness of the man lay in his ability to take the idea of non-violence and make ordinary people do extraordinary things.''

Wilson, 63, who also is black and has been professor of art at Boston University for more than two decades, never met King but drew his impressions from photographs and modeled his study after contemplative figures from the art of Egypt, the Far East and Africa.

''I saw in King the quiet, profound, universal serenity of the inner- directed thinker, rather than the image of overt action, the sort of image you can read at a glance, like a photograph or a poster - the clenched fist, the open mouth, the outstretched arm.''

Wilson's approach was endorsed by Edmund Barry Gaither, director of the museum of the National Center of Afro-American Artists in Boston. Gaither was chairman of the review panel of the National Endowment for the Arts which selected Wilson's design from among more than 200 entries in a national competition.

''This is an extraordinary work in both the psychological as well as the artistic sense,'' Gaither said. ''It is heroic rather than descriptive. It represents not just one action - the King of the march on Washington, the letter from the Birmingham jail or the Nobel Peace Prize - but the King of all time, the King of deep thought and reflection and contemplation. ''

Wilson saw Mrs. King after the bust's unveiling and said ''she seemed to feel it was adequate.'' But Gaither said King's widow, who met with Wilson twice during his work on the sculpture, ''not only was pleased but was enthusiastic about the image that was caught.''