The Rev. Robert L. Pruitt was disappointed with the lack of spirit in the audience yesterday as he stepped to the rostrum to give the keynote address at the city's official commemoration of the 49th birthday of slain civil rights leader Martin Luther King Jr.

"I look at some of you, and all we need is a corpse out here and we'd be satisfied," said Pruitt, pastor of Metropolitan AME Church in northwest Washington. The staid mood seemed somehow improper, Pruitt thought, for a person like King, who was "a man of great emotion.

So Pruitt began to preach.

In a style laden with the same kind of symbolism and analogies King often used in speeches that gave beaten, tear-gassed and discouraged civil rights workers renewed spirit during demonstrations in the South, Pruitt praised the late Baptist minister.

He said King, by his deeds, had turned "trash into triumph, tragedy into victory and death into life." He criticized commercialism of King's birthday, warning that it should not become "just another excuse for merchant princes to capitalize on this prince of peace."

Pruitt's voice rose and fell in deep, sometimes throaty tones. And the audience, which was soon regularly filling his pauses with shouts and amens, was at perhaps the peak of its emotion when Pruitt made the inevitable comparison between King and Sen. Hubert H. Humphrey (D-Minn.). who died last week in Waverly, Minn.

"It must have been a great moment late Friday night," Pruitt said, "when Martin Luther King moved onto the banks of the Jordan River to meet Hubert H. Humphrey as he crossed over.

"I can hear Hubert saying to Martin, 'I thought I was free when I received my degrees and I was called a specialist in economics. I thought I was free when I moved to the Senate and (when I) became mayor of Minneapolis.

"'But I only know what freedom is now, when I drop this robe of clay and put on the garment of spirituality. I'm free at last. Thank God Almighty, I'm free at last.'"

Then Pruitt added, "I can see him look down from heaven into the great pits below as he looks at (former FBI Director) J. Edgar Hoover and says, 'I've done some right and I've done some wrong. Now I must go, where I belong.'" (At Hoover's behest, the FBI had carried on a campaign to harss and discredit King.)

Pruitt's speech to the audience of more than 700 persons at the Martin Luther King Library in downtown Washington capped a two-hour memorial service for King, whose assassination during a sanitation workers' strike in Memphis in April, 1968, plunged this city into days of rioting, looting, burning and death.

Yesterday's memorial was closely linked with the death Friday of Humphrey, a strong advocate for civil rights. The affair was a time for remembering what King had done in the past and suggesting what might be done in the future to serve his memory.

Comedian and civil rights activist Dick Gregory recalled his own feeling when he noticed that a street named in his honor in his hometown, St. Louis, intersected another street named in honor of King.

"The most important thing about that street is that Dick Gregory Street ends at Martin Luther King Drive," Gregory said. "That's how my life changed, when Dick Gregory bumped into Martin Luther King."

Gregory said that the civil rights movement led by King had given the nation and education and measure of progress that he felt could not be found in schools. "This movement has contributed something to America that's been missing for a long time," he said. "It's called integrity, ethics and honesty, and when you add that to reading and writing and counting, things change.

Mayor Walter E. Washington sat on stage with City Council Chairman Sterling Tucker, other Council members, members of the D.C. Board of Education and other city officials. The city government had declared yesterday an official city holiday.

Referring to King's dream of a racially integrated America, which the late civil rights leader had talked of in a famous speech during the 1963 March on Washington, the mayor said, "The dreamer has gone, but the dream persists, or else we belie what we see in the eyes of these young people. . . "

The mayor used the occasion to ask Congress to declare King's birthday a national holiday, to pass a full employment and public works bill cosponsored by Humphrey (the Humphrey-Hawkins bill), and to grant the city "full home rule."

"Just pass the bill and take the yoke off the city," Washington said."And then recess for Easter and pray, because the Lord will abundantly bless you."

Pruitt lauded King's concern for others and the late civil rights leader's belief in the supremacy of human dignity. He said King was one to whom "it was difficult to find how a man of 45 could be called a boy."

"How is it." Pruitt asked, "that a man who held to such a philosophy -- nonviolence -- could be handled like trash?"

There were many children and young adults in the predominantly black audience, which filled every seat in the room and stood laced through the aisles between card catalogues in the new, brick and glass panel walled lobby.

Near the end of the memorial, after the Woodson Senior High School Male Chorus had sung the freedom song "I Shall Not Be Moved," the audience members stood, crossed arms, joined hands and swayed back and forth singing "We Shall Overcome."