Major civil rights groups broke off their uneasy relationship with the U.S. Commission on Civil Rights yesterday as Rep. Parren J. Mitchell (D-Md.) accused its chairman, Clarence M. Pendleton Jr., of being an administration "lackey" for calling some black leaders "immoral" and "racists."

Mitchell, whose family has for decades been at the forefront of moves for racial equality, walked out of a commission hearing, at which he was to testify, after protesting a speech by Pendleton on Tuesday in which he criticized civil rights leaders for promoting "a new racism."

"You neither deserve my response to any question, nor do you deserve any recognition, nor do you deserve any respect," said Mitchell, who joined the leaders of major civil rights groups in a boycott of the two-day hearings on affirmative action in employment.

"He has chosen to play a lackey role. And if he wants to be a lackey for those who would crush black aspirations, fine," the Baltimore congressman told reporters. "However, if he's chosen to play that role, he will not play it with my support."

Pendleton appeared unruffled by Mitchell's remarks, but commission vice chairman Morris B. Abramshouted at Mitchell as he was leaving the packed hearing room: "I'd like to ask the congressman a question. Why did he accept an invitation to come? Why did he submit testimony and run?"

Abram said the boycott by civil rights groups represented "a historic division point" and a "sad manifestation of the state of a once glorious movement which celebrated openness and fairness."

"Their action is petulant and unworthy of the founders of the civil rights movement," he added.

The boycott represented a major departure for a host of civil rights groups, including the National Urban League, the NAACP, the NAACP Legal Defense and Education Fund, the National Organization for Women, and the Mexican American Legal Defense and Education Fund.

It reflected growing frustration and displeasure with Reagan administration policy on racial issues, which the groups maintain has become more strident since the November election. Although the groups have criticized the commission in the past, they had continued to cooperate with it until yesterday.

A statement issued by several of the groups said recent events make it evident "that a majority of the Civil Rights Commission has become public advocates against the remedies for discrimination" -- such as goals, timetables and quotas -- that were the focus of the hearings.

"These sessions are like an Alice in Wonderland event: first the verdict, then the trial," the statement added.

The civil rights groups noted that Pendleton and Abram told President Reagan in January that the commission had made quotas a "dead issue" and replaced the debate over increasing preferential treatment for minorities and women with a vision of a "colorblind society."

In his Tuesday speech at the National Press Club, Pendleton, who is black, accused "media-designated black leaders" of promoting racism because "they treat blacks differently than whites because of their race."

Pendleton said yesterday that he regretted that Mitchell and civil rights leaders had chosen not to testify before his panel. "They do a disservice to the American public," he said. "The public deserves to hear what is the value of the affirmative action program."

Mitchell and several civil rights leaders submitted written testimony. In addition, academics and spokesmen for business groups appeared in sessions that lasted into the evening. The most heated testimony focused on the role of civil rights groups and the commission.

Walter E. Williams, a George Mason University economics professor, said the old civil rights struggle "is over and won" and "the new civil rights movement is an effort by some to impose greater government control as a means to acquire more personal political power and wealth."

David H. Swinton, director of the Southern Center for Studies in Public Policy at Clark College in Atlanta, charged that the commission has abandoned its role as an advocacy group for minorities. "This commission was not established to be neutral or to promote the interests of white men," he said. "They don't need help. They control everything."