The idea of Clarence Mitchell Jr., the dauntless lobbyist behind all of the modern civil-rights legislation, retiring amused most of the 1,000 guests who turned out for his 68th birthday party last night.

Sen. Birch Bayh (D-Ind.) shook his head in disbelief. NAACP attorney Nathaniel Jones said Mitchell hasn't yet learned he no longer has to punch the clock.

Actually, what made his retirement after 30 years as the NAACP's chief lobbyist on Capitol Hill hard to comprehend was the fact that he has been there longer than almost anyone else. Mitchell, the elder of a prominent Maryland civil-rights family, had first stepped on Capitol Hill to testify about a lynching he had witnessed in 1933.

In between the interviews and the picture posings last night, it was those who had learned from Mitchell who tried to grasp their loss. "He educated us. His voice could shake the rafters, but what occurred was a thorough, sensitive lesson," said Sen. Edward Kennedy (D-Mass.), who experienced Mitchell's strategy sessions on the poll tax as a freshman senator.

When G. Harrold Carswell was nominated to the Supreme Court, Bayh recalled, "I was ready to forgive his remarks, 'I yield to no man in my belief in white supremacy,' but Clarence looked around the room and said. 'I don't know about the rest of you but a man who once believed that can't believe in the basic truths of this country.' Clarence taught me some basic lessons."

One pupil who sent his regrets for the birthday celebration at the Washington Hilton Hotel was Vice President Walter Mondale. He called in sick at 8 p.m.

Among the guests were former senators Hugh Scott and Dick Clark, Sens. John Glenn (D-Ohio) and Paul Sarbanes (D-Md.), presidential assistant Louis Martin, Washington Mayor Marion Barry, Reps. Charles Diggs (D-Mich.) and Cardiss Collins (D-Ill.), civil-rights leaders Dorothy Height and M. Carl Holman, EEOC Chairman Eleanor Holmes Norton and the AFLCIO's Lane Kirkland.

The tribute was sponsored by the National Office of the NAACP and the money raised, which executive director Benjamin Hooks estimated at $75,000 gross, is to go to the organization's depleted treasury.

At one point in the evening, Juanita Mitchell, an attorney and civil-rights leader in her own right, led the couple's four sons and 10 grandchildren in a round of tributes. Mitchell's youngest brother, Rep. Parren Mitchell (D-Md.), recalled, "When I first came to Congress Clarence and I sat down and he said, 'Now you are the congressman. I will never attempt to interfere with what you do.' He didn't have to because I had been following his example all along."

The battles with the segregationists were vividly recalled. Joseph Rauh, a long-time civil-rights lawyer, recalled, "When we were fighting for the 1957 civil-rights bill, Clarence and I were sitting in the House gallery late one evening. Harry Byrd was on the floor, not this young peanut but the old man, and he pointed up at us, 'There go the golddust twins.' Being his fellow traveler hasn't been easy."

Though Mitchell is a flgure of dignity and restraint, he has another side. One old friend, Gloster Current, recalled the time Mitchell dragged an intruder out of a convention, and when the intruder broke from his grasp Mitchell smashed the window of his car. "Now that was not a nonviolent act."