The Justice Department yesterday asked a federal appeals court to overturn as unconstitutional a court order requiring Detroit's police department to promote equal numbers of blacks and whites to the rank of lieutenant.

Assistant Attorney General William Bradford Reynolds, head of the department's Civil Rights Division, has called such plans "morally wrong" because they use race as a consideration in the hiring or promotion of persons who were not necessarily victims of discrimination.

A three-judge panel from the 6th U.S. Circuit Court of Appeals in Cincinnati ruled on March 29 that Detroit's promotion system is legal. But, in a brief filed yesterday in that court, Reynolds asked that the full court reconsider the decision and that the Justice Department be allowed to enter the case.

The "one-to-one racial quota for promotions to the rank of lieutenant impermissibly infringes on the equal protection rights of non-black police sergeants . . . . The right to be free of unlawful racial discrimination in employment belongs to individuals, not groups," Reynolds said.

"The one-to-one promotion quota . . . clearly embraces and benefits non-victims as well as victims of . . . past unlawful discrimination and, thus, accords racially preferential treatment to persons having no 'rightful place' claim to promotion priority vis-a-vis non-black officers," he said.

The Detroit case is the third major court challenge by the Justice Department under the Reagan administration to affirmative-action plans to remedy past racial discrimination.

The department has asked the 5th U.S. Circuit Court of Appeals to reject a court-approved consent decree under which the New Orleans Police Department must promote one black to an officer's rank for each white promoted.

The Justice Department also has asked the Supreme Court to overturn a court-ordered plan under which Boston's police and fire departments made layoffs on a racial rather than seniority basis to protect minority employes hired recently under an affirmative-action plan.

Unlike those two cases, the Detroit plan was not ordered by a court. It was adopted voluntarily by the city in 1974 after a long history of antagonism between the mostly white police department and the black community.

When white police officers challenged the plan in 1975, U.S. District Court Judge Damon Keith not only upheld it, but incorporated the affirmative-action plan into a court order.

The three-judge appeals panel ruled unanimously that the plan is constitutional and permitted by Title VII of the 1964 Civil Rights Act, which prohibits employment discrimination. The panel also found by a 2-to-1 vote that Keith acted properly in incorporating the plan into a court order.

Reynolds argued in his brief that the District Court exceeded its authority by incorporating the plan into a court order. Even if it had remained voluntary, Reynolds said, the plan "constituted an inequitable infringement on the interests of innocent non-black candidates for promotion to lieutenant and violated the equal protection guarantees" of the Constitution.

The Supreme Court, which is expected to make a decision soon in the Boston case, has never confronted this precise issue.