D.C. Mayor Marion Barry announced yesterday that he will bow to congressional demands and hire 200 new police officers. He said, however, that rather than selecting the top 200 scorers on the most recent recruitment exam, the city will choose candidates by lot from a pool of 635 applicants who passed the test in order to increase the numbers of eligible blacks and women.

In announcing the decision, Barry said the highest scores on the March 28 exam were attained disproportionately by whites and males, leaving the city open to possible lawsuits charging discrimination.

The plan to draw the names at random drew immediate criticism from the International Brotherhood of Police Officers, the bargaining agent for the department's rank-and-file officers.

"I think it's very unfair," said Larry Melton, vice president of the union. "They're changing the rules on these guys. If this was what they were going to do, then this should have been in the ground rules from the beginning." However, Melton said he doubted the union would attempt any legal action against the city.

In addition to scheduling the random drawing, the city has lowered the passing score on the exam to include more blacks and women in the pool of 635 candidates. Previously, a score of 40 correct answers out of a possible 80 was a passing score. Officials decided to lower the pass level to 35.

The reason for adjusting the test results, Barry said, was that the original list of candidates ended up top-heavy with white males. While more than 75 percent of the 838 persons taking the test were black, Barry said, 62 of the top 100 scorers were white. About 24 percent of the applicants were female, but only 13 of the top 100 scorers were women.

The new procedure, Barry estimated, will end up giving about 70 percent of the police jobs to blacks and about 23 percent to women.

"Without certain adjustments, using the results . . . to select new police officers would violate Title VII of the Civil Rights Law of 1964," Barry said in an executive order spelling out the procedures for altering the original list. "If it was used as it was in the past, the test would have a severely adverse impact on minorities and women."

Barry said the drawing will be held next Monday in the D.C. City Council chambers. The city's personnel office will provide Police Chief Maurice T. Turner with the new list of applicants by Sept. 1, and Barry said the additional 200 officers can be hired by next February.

Turner said, however, that because of a four-to-one dropout rate usually encountered among applicants, the list of 635 persons might not be enough to produce the 200 officers that the city wants to hire. The city may have to administer the exam again, he said, which would likely cause further delay.

Turner said the additional officers will allow him to implement a plan to assign 40 plainclothes officers full time to combating the city's mounting illicit drug problem.

Barry's decision to hire the officers appears to end a year-long battle with the influential House D.C. Appropriations subcommittee, whose members directed the city last November to increase the number of D.C. police officers from 3,600 to at least 3,800. Current strength of the force is about 3,630 officers.

Barry repeatedly told the subcommittee, which controls the District's purse strings, that he intended to hire the extra officers, for whom the panel gave the city an extra $6 million. But he repeatedly told reporters at press conferences that city officials, and not Congress, should decide how many officers the city should have. And in drawing up his budget for the fiscal year beginning Oct. 1, Barry included only enough money to pay 3,621 officers.

Barry and his aides continued to tell the subcommittee that suspected bias against blacks and women in the test results had delayed the planned hirings. As the months went on, however, members of the subcommittee became more and more impatient.

Finally, the subcommittee last month gave its mandate some muscle by refusing to agree to a routine transfer of funds from the Police Department to the Department of Corrections. The action threatened to thwart Barry's often-repeated promise of balancing the District's budget this fiscal year.

Within a month, Barry had decided that adjustment of the March test results would solve the problem of bias. And Barry wrote a conciliatory letter received July 29 by Rep. Julian Dixon (D-Calif.), chairman of the subcommittee, saying that the hirings would take place as soon as possible.

" . . . I am troubled about the perception the commitee has gotten about our commitment to hiring police oficers in FY 1981 as directed by the committee," Barry wrote. "This perception can be tied in large measure, I believe, to a failure on my part to keep the committee fully informed on some of the technical aspects related to carrying out the committee's directive."

Barry wrote that he had been unaware, at the time of the congressional mandate, that responsibility for administering the police examination had passed from the federal Office of Personnel Management to the city government.