AFTER COMEDIAN Richard Pryor stepped to the bank of microphones at the Agriculture Department and--as if he couldn't resist one joke--bent to lick them like ice cream cones, he stopped being funny.

He wasn't laughing, or smiling his quick radiant smile, or saying dirty words. Instead, several times, he wept openly as he delivered a sober and, in places, impassioned speech in a moving commemoration of Dr. Martin Luther King Jr. before 1,500 federal workers yesterday.

"Oh, if you are black in America, you got a problem," he said. "We all have a problem . . . We got a problem!" He said that while "there is a great feeling . . . that all white people are bad," this was not true.

On the other hand: "There are millions of evil white men out there who control the society, call me a nigger." Applause.

"And you and I know there are no niggers." Applause.

"Niggers are gone forever!" APPLAUSE.

"Now we are free, and that brings about a responsibility. We are free to starve to death . . . If you do better your condition, don't forget to look over your shoulder, reach out your hands and pull someone else along with you!"

At this, there was a roar of applause and cheers--the biggest one of the day. Pryor appeared to be caught up in the emotion of his audience, and he declared that "Dr. Martin Luther King's holiday has got to come out of the closet and become a national holiday."

Pryor came to Washington Thursday because an Agriculture Department employe, Armstrong Williams, thought to ask him. Yesterday he gave his speech and made courtesy calls on the Hill, including one to Sen. Strom Thurmond, Williams' friend and mentor.

Pryor plans to attend a White House reception for the anniversary of King's birth today, then catch the red-eye back to the West Coast.

"I don't make speeches," he said in his speech yesterday, occasionally stumbling over the straight lines. "I'm a professional comedian . . . and as you can see, I can't even talk very good when I'm not making jokes."

The members of the audience didn't agree. They sang "God Bless America" and "Lift Every Voice and Sing" and then they cheered and applauded as Pryor explored--quietly at first and then with building intensity--the wrenching theme of racism in American life, a theme so familiarly, savagely, tenderly dealt with in his humor.

Pryor began his speech on a confessional note, saying he never rode the freedom buses, never went south to fight for civil rights. "I was too frightened to go down to Mississippi . . . I was too frightened to be a part of that."

Hence his enormous admiration for those who did go, led by King. "It swelled my heart up with joy and pride to think of the courageousness of every person who participated in that time . . . If it weren't for you people who went down there and did that, believe me, I know in my heart that I wouldn't be here today."

At this, Pryor broke down briefly and wept, pulling a handkerchief out of his pocket to wipe away the tears. As he went on to describe King as a great man of faith and hope, Pryor wiped his face again from time to time.

He called on black Americans to strengthen their ties with Africa, the "motherland," and took a swipe at American liberals: "Black people of America, times have changed. The white liberal so-called establishment have other games to play . . . the nuclear freeze . . . El Salvador . . . They have moved on, because in the 1960s when we got militant we told them we didn't need them, and they took us at our word."

After the ceremony, which included prayers and a singing of spirituals, Pryor was mobbed as he left the dais. He had to be protected by a phalanx of uniformed security guards, who rushed him to a nearby conference room. There he held a brief press conference, saying the speech was his first straight speech. "And my last."

He added, "I'm just doing this for Dr. King today. Today only."