For the first time since 1975 the D.C. police department is setting up a recruiting office and will hire 50 new officers in the coming weeks.

The department's manpower, currently at 4,100, had dropped steadily since it peaked at 5,070 in 1972. Under the city's 1977 fiscal budget, which is still operative since Congress has not yet given approval to the 1978 budget, the department's authorized strength is 4.150.

"I received authorization from the chief three weeks ago to go ahead and hire 50 people to get the force back up to its authorized strength," Waddell Longus, the departent's director of personnel said yesterday. "I don't know exactly when we'll fill the 50 spots but we do have over 600 applications on the file right now."

In submitting the city's 1978 budget to the House of Representatives District Appropriations subcommittee, Mayor Walter E. Washington requested a further reduction in the department's manpower, asking it be cut by 86 men. The City Council also approved reducing the manpower by 86.

After former police chief Maurice J. Cullinane told the committee in April that a reduction in manpower would lead to an increase in the city's crime rate, committee chairman William H. Natcher (D-Ky.) urged that the force not be reduced.

As a result the current budget pending before Congress calls for no reduction in police manpower.

The number of officers on the police force has dropped steadily since 1972, except in 1975, the last time the department did any hiring. The authorized number of officers also has dropped, from 5,100 until 1974 to 4,750 in 1975-1976 to 4,150 in 1977.

The new recruiting office is being set up in the storeroom in which the tear gas canisters that exploded on Jan. 12 were kept. Longus said yesterday he did not know when the office would be ready for use, but said operations would begin as soon as possible.

Longus said he expected eight persons to work in the recruitin office and that their first job would be to "go through our current applications and come up with the best people available for the department." He said he did not yet know how many woman would be included among the new officers.

A police spokesman said yesterday that chief Burtell M. Jefferson, who succeeded Cullinane on Jan. 13, felt confident in authorizing the hiring and the recruiting office since the 1978 budget calls for manpower to remain at 4,150.

He added that the department felt there was a need for the recruiting office since, "We'd have to keep hiring at a pretty rate to keep our manpower where it is with attrition being what it is today."

He said the City Council, the mayor's office and Congress all had been informed early this month of the decision to hire the new officers.

The police department's manpower has been a subject of controversy ever since 1969, when its manpower began increasing largely because of FBI crime surveys that rated Washington the No. 1 city among 20 surveyed in its crime rate.

Manpower increased steadily from 4,436 in 1969 to 5,070 in 1972 before the job freeze was instituted in 1972. Washington has steadily dropped in the crime survey, reaching 17th last year.

Since that time the city has dropped to 17th in crime among the 20 cities rated. Supporters of the police department have claimed this is because of the increase in the city's police force.

Others say that the 5,000-member force was much too unwieldy and felt that a number under 4,000 would not be impra ctical in a city with a population of 705,000.