Police Chief Burtell M. Jefferson warned yesterday that sharp reductions in police personnel now being considered by Mayor Marion Barry would "pose severe and grave danger to the residents of the District of Columbia."

Barry already has ordered the elimination of 120 jobs from the 4,300-member department by Sept. 30, and is contemplating another 200 layoffs chiefly of uniformed officers by the end of the year.

In a confidential memorandum personally delivered to City Administrator Elijah B. Rogers yesterday, Jefferson said such personnel cuts could force reductions in high visibility police patrols "potentially resulting in increased incidents of street robberies, larcenies and burglaries."

Jefferson's memorandum, which echoed the concerns over potential budget cuts that many city department heads have voiced in private, warned of other adverse effects from Barry's attempts to balance the city budget, Jefferson said:

Emergency calls on the special "911" telephone number would have to be stacked and responded to more slowly. About 2,700 such calls are received each day.

Proposed reductions in overtime funds for the department could cripple the city's effort to police large demonstrations, such as last year's by Iranian students.

Plans to save money by increasing the number of patrol cars manned by one officer rather than two would jeopardize the safety of affected officers.

At about the same time that Jefferson was delivering his memorandum, representatives of an organization representing 250 higher ranking officers on the force were holding a press conference in the plaza outside police headquarters.

Capt. Max J. Krupo, president of the Metropolitan Police Officials Association, told reporters that the budget cut were hurting morale in the department. Krupo said his organization will lobby City Council members to take action against the personnel reductions.

"We . . . call on the citizens of this community to stand by us, and prevent this nightmare from becoming a reality," Krupo said. "The relative ease with which the recommendations" for layoffs were made "has forced our members to question the value of ur services."

Krupo contended that over the years the police department, unlike other city agencies, has stayed within its budget.

"Our money is . . . used to help sustain the overspending of others," Krupo declared.

On Wednesday, Budget Director Gladys W. Mack proposed a package of spending reductions to be implemented between Oct. 1 and Dec. 31 in an effort to help balance the city's budget.

Barry is still considering Mack's recommendations, which include 426 job reductions in the police department, about 300 of which would be done either by layoffs or reductions by attrition. The remainder would take place through abolishing job slots that currently are vacant.

Ranking aides to the mayor have said Barry may consider some changes within departments, but will probably accept Mack's general recommendations to eliminate nearly 3,000 jobs from the city payroll by the Dec. 31. Barry already has ordered the elimination of 1,540 jobs by Sept. 30.

It is during the time that the minor changes are under consideration that city department heads usually lobby the mayor to encourage smaller cuts in their departments.