he police head-quarters building here is a grime-gray relic dating to 1921. Aside from a new, NASA-like computerized command center on the third floor, installed for last year's Republican convention, the structure appears to have stood undisturbed for decades, static and unchanged.

The appearance deceives: There have been enormous changes in the Detroit police force, changes at the center of a bare-knuckles turf fight over police jobs and the status and power they bring.

Detroit, like Washington and most other major American cities, has seen an enormous influx of blacks and other minorities. But the uniformed services -- the police and fire departments -- of these major cities until recent years remained largely white. Frequently these jobs were effectively passed down through white ethnic group families from grandfather to father to son.

Largely within the past 10 years, as blacks and minorities have gained political power in some cities and the courts have intervened in others, the numbers of blacks and Hispanics in these uniformed services have increased. Often the process has involved extraordinary affirmative action measures.

Many white officers, who vigorously opposed the influx of these minorities at first, have come to accept the phenomenon because it often makes it easier for them to operate in minority communities -- "operational need" is the euphemism most often applied. But many of them still do not believe that affirmative action programs that bypass whites for jobs or promotions are fair.

D.C. Mayor Marion Barry recently encountered sharp criticism when he announced that he would adjust the results of a March 28 police entrance examination to include more black candidates for admission to the police academy. Barry lowered the passing score on the test from 40 to 35 (out of a possible 80), and then picked academy candidates by lot from the list of those who passed under the new guidelines instead of by test score ranking.

The issues in Washington, where the test is alleged by Barry to be culturally biased against blacks who make up 70 percent of the city's population but only 45 percent of the police force, are similar to those in other cities.

New York Mayor Ed Koch, under federal court order, last year also lowered the passing score on his city's police entrance exam -- but only for minority candidates. The city's 23,000-member police force contains only about 14 percent minorities while about 45 percent of the city's population is made up of minorities. Under Koch's guidelines, minorities were allowed to pass the test with scores 10 points lower than the passing score for whites.

New York City gave another entrance exam this year, with the same passing score for all candidates. But to ensure that minorities are selected, Koch agreed to pick candidates for the police academy by lot from the pool of those who passed the test, similar to the way it was done in the District.

In Atlanta, Mayor Maynard Jackson, the city's first black mayor, was pressured into removing Reginald Eaves as public safety commissioner after Eaves was accused of giving blacks the answers to police examination questions. Eaves, who subsequently became a Fulton County commissioner, is now running for mayor; and while he does not lead in the polls, he is perceived to have a base of support in the city's black community, where the examination episode tainted him less than among white voters.

Lawsuits over the integration of the uniformed services have been filed in Philadelphia, Los Angeles, Chicago, Miami and other smaller cities, and the battle rages in county and state security forces as well.

But nowhere, perhaps, is the dispute more clearly drawn than in Detroit, where the gleaming new 70-story Renaissance Center mocks a vista of relentless urban decay. Detroit, long known as the Motor City, is beset by crushing unemployment stemming from the auto industry's malaise, so police jobs that start at $21,183 understandably do not go wanting for applicants, both black and white.

The city is 62 percent black and the police force is now about 35 percent black. That's up from 15 percent eight years ago but down (because of layoffs forced by city budget reductions) from a high of over 40 percent three years ago, according to Detroit officials.

"Police forces have really been political instruments around the country," said Barry Goldstein, a lawyer with the Legal Defense and Education Fund. Goldstein defended the city of Detroit against two lawsuits filed by the white-dominated Detroit Police Officers Association. Both challenged the city's new policy of promoting one black officer for every white officer promoted, regardless of test rankings.

Police forces, Goldstein said, "were the way to power for white ethnic groups. They were political. And in being political, and being ethnically centered in some places, by definition they were discriminatory. Minorities were just not in the group."

Police jobs generally pay well, provide steady work and carry status and position. Just as white ethnics -- Italian, Irish, German -- did before them, blacks and Hispanics wish to use them as a tool for advancement. But Goldstein said there are other reasons to bring more minorities onto police forces in cities where minority populations have increased dramatically.

"The other side of it is that in minority communities around the country, the most important, most visible, in many cases the only government presence in the community is the police," he said. "If you look at it pejoratively, they are the army of occupation. If you look at it a different way, they are the social workers. However you look at it, they are there on the streets . . . . And they have an enormous amount of power -- life and death."

Mayor Coleman Young, Detroit's first black mayor, recalls that between his election in November 1973 and his swearing-in the following January, he attended three police funerals as mayor-elect. "I remember them clearly," he said in a recent interview. "I had to buy some long underwear to stand out in the cold."

His first year in office he went to the funerals of two more police officers killed in the line of duty, Young recalls. But there hasn't been a Detroit police officer killed in action since.

Young attributes that winning streak at least in part to the addition of more blacks to the police force -- an assertion that is is difficult to prove. Young made integration of the police force a major campaign issue, and promised to create a half-black, half-white police department.

In fact, the city did achieve a turnaround. In 1973, the year before Youngg took office, 70 percent of the new police officers hired by the city were white and 30 percent were black. In his first year, 1974, the numbers were 52 percent white and 48 percent black. The year after, 64 percent black and 36 percent white.

In 1978, the last year in which the financially distressed city government actually hired new officers, 77 percent of them were black and 23 percent white, according to figures provided by the police department.

"In the election, we raised the whole question of the relationship between the police and the population, the black population," Young said. "Detroit had explosions in 1966 and '67 which almost without exception were triggered by incidents between white police and the black community. That was the spark in which alienation and hostility exploded."

Detroit's 1967 riot was one of the most destructive of the disturbances that shattered the nation's cities in the late '60s. A number of studies, including the Kerner Commission report, has cited the paucity of blacks on the city's police force as a contributing factor to the tensions that led to the riot.

In Washington, by contrast, Barry encountered a different situation and took a different approach. In announcing two weeks ago the adjustment of the examination test results, Barry said he was doing so because Title VII of the Civil Rights Act compelled him. Barry's political base, unlike Young's, lies outside the black community, and integrating the police force was not a major campaign issue.

The District's police department already had a much better hiring record of blacks than did Detroit's. The 1968 riot in Washington resulted in only two demonstrators being killed by police, as opposed to about 40 in Detroit. The police force was dramatically expanded between 1969 and 1971, and many of those new officers were black.

Detroit's major initiatives were to recruit blacks actively; to enforce and then toughen a residency law whose laxity had allowed suburban whites to join and remain on the force; to remove perceived bias from police examinations; and, most controversially, to institute the policy of promoting one black officer for every white who got promoted.

Recruiters cruised the city's depressed black neighborhoods in two Dodge vans, signing up potential applicants. Those who did apply were urged to invite their brothers, sisters and cousins to do the same. Prospects who signed up but did not follow through were called immediately in an attempt to persuade them to stay on.

The toughening of the residency laws, meanwhile, increased the number of minorities in the applicant pool by making it much tougher for officers to get away with living in predominately white suburbs. Now, anyone who wants to be a Detroit policeman must live in the city for two months before applying. The percentage of black applicants to the Detroit police force rose from 54 percent the year before Young took office to 81 percent the year after.

The District requires newly hired employes, including policemen, to move into the city within six months.

A four-hour, general knowledge entrance examination, generally similar to the one given in the District, was replaced by a more streamlined, job-related test before Young took office. But he changed the test again after concluding that it still contained bias against minorities, changing the promotional exams as well. The percentages of whites and blacks who pass the tests are now virtually identical -- a crucial test of bias in the eyes of the courts. Barry said that by that standard, the District's current tests are also biased.

On the recent test administered in the District, before the test results were adjusted, 94 percent of the whites who took the exam passed while only 54 percent of blacks passed.

The biggest fracas in Detroit erupted over the new policy for promotions. "We became very arbitrary when it came to promotions," Young acknowledged, simply promoting one black for every white.

The decision to do so resulted in the white-dominated bargaining agent for the rank-and-file promptly filing suit. The police union charged that Young's policy of passing over some whites who had scored highly on promotional exams to hire some lower-scoring blacks violated their rights. The group won a favorable decision in U. S. District Court in one suit but that was subsequently overturned in the U. S. Court of Appeals. The union has appealed an unfavorable court ruling in its second court action.

"I think the feelings were higher than the lawsuits might indicate," said Cmdr. Chuck Henry, whose office is responsible for monitoring the success of the minority program. White officers, mindful of a rule against criticizing department policy, are reluctant to talk publicly about the promotional procedure. Most of those interviewed said privately that they had vigorously opposed the policy at first, although some said they have since come to accept it.

"It cost me a promotion," said one white officer. "I was on the list but they passed me over for sergeant . I figure over the period of the last five years it's cost me twenty grand. I was bitter about it. It created some bad feelings. In terms of community relations, it might be good. But there were a lot of guys in my position. You're No. 110 on the list and all of a sudden somebody's your supervisor who was No. 400 on the list. That created some bad feelings."

The officer concluded, however, that integration of the force was "a good thing" because it makes it easier for the police to operate in the city's black neighborhoods. In Washington, too, white officers have generally accepted the operational validity of having more blacks on the force.

William L. Hart, the black police chief appointed by Young, calls the Detroit police force as well as all others in the country "among the most conservative institutions in America -- semimilitary and resistant to change." Yet, despite that, inflamed feelings have cooled, and many believe that is partially due to the fact that the cash-strapped city hasn't been hiring or promoting any officers for more than two years now.

The layoffs of 1,000 Detroit officers have slowed -- but not stopped -- affirmative action, Hart said. The city is losing about 300 officers a year to attrition, mostly retirements, and these tend to be older, white officers. When they leave, laid-off blacks replace them.

In Washington, there is also a fight over further integrating the city's fire department, now about 31 percent black. Anita B. Shelton, city human rights director, last week ordered the city to insistute a wide-ranging affirmative action plan and to fill all but 10 of 70 current openings with minorities.

There is a fire department fight in Detroit as well. Last year voters approved for the first time a promotion system based on merit examinations. Currently, fire department promotions are handled strictly by seniority, with supervisors required to evaluate their subordinates when they come up. Historically, according to Detroit's Fire Chief Melvin D. Jefferson, that meant that white supervisors gave excellent ratings to white subordinates while blacks were left out.

Jefferson, who is black, said the system will be changed next year. "It's a bad system," he said. "A backward system."