The Justice Department filed a complaint yesterday against the Milwaukee Police Department, charging that it discriminates against blacks in promotions and excludes blacks from assignments in white neighborhoods.

The complaint was filed in U.S. District Court in Milwaukee by William Bradford Reynolds, head of the Civil Rights Division, who has come under widespread criticism for his civil rights enforcement policies.

Reynolds recently called mandatory school busing and affirmative-action quotas the equivalent of a "racial spoils system." His division also recently filed motions in support of whites in Boston and Detroit protesting quotas that favor blacks.

In the Milwaukee case, Reynolds asked the court not for a quota but for an order that the city discontinue discriminatory promotion examinations and promote with back pay and seniority blacks who took the exams in the past but were unfairly denied promotion. It also asks that the discriminatory assignment system be ended.

The suit charged that blacks have been all but excluded from promotions to sergeant and detective and that no black has ever been promoted to lieutenant. In addition, the Justice Department charged that no black police officer has ever served in the two police districts that cover the city's white neighborhoods.

A 1974 Justice Department suit against the Milwaukee police charged racial discrimination in entry-level jobs. That lawsuit was settled in 1975 by a consent decree.

The new complaint was filed just a day after President Reagan defended his record on civil rights at a news conference. He charged that he has been the victim of a "hatchet job" and said black voters would support him "if we could get the truth to them."

Reagan's statements were immediately criticized by civil rights leaders and some of his figures were contradicted by federal records.

Ralph Neas of the Leadership Conference on Civil Rights, an umbrella group of 165 groups representing blacks, Hispanics, women, organized labor and the elderly, said yesterday of Reagan's remarks, "That is an astonishing misrepresentation of the Reagan administration's record on civil rights.

"This administration, unless it changes course dramatically, will be remembered for its systematic attempts to weaken our civil rights laws and dismantle the government enforcement apparatus."

Reagan said that he was being hurt by "perceptions that somehow our budget cuts have affected the black community more than any other."

Eddie N. Williams, president of the Joint Center for Political Studies, said, "His Cabinet is doing the hatchet job in terms of some of the policies they are espousing. In every index we have looked at, reviewing the policies of this administration, blacks, other minorities and other low-income people are disproportionately affected."

In defending his commitment to black voters, Reagan said that under his administration the Justice Department has "conducted some 21,000 inquiries into . . . what we think are suspected voting violations."

But congressional testimony last week by Reynolds indicates that since Reagan took office, the division has objected to 134 changes in voting procedures and filed 25 new lawsuits based on the Voting Rights Act.

The 21,000 figure was actually the number of voting-procedure changes submitted to Justice for its required approval. It was abnormally large because of widespread changes in voting districts required by the 1980 census figures.

Reagan also was criticized for taking credit for last year's extension of the Voting Rights Act.

"The administration led the fight for many months against a strong extension of the Voting Rights Act," Neas said. "Indeed, even when it became evident that Senate leaders had worked out an agreement, top Justice Department officials did everything possible to scuttle the compromise. Only when confronted with a fait accompli . . . in the Senate did the administration decide to endorse the legislation."