She said the officer was in uniform working at a part-time security job at an apartment house around 6:45 p.m. when he encountered a group of people, described as males, in the 300 block of Anacostia Road SE.
A man encountered by the officer had a handgun, the official said.
Few other details were available. It was not clear whether a gun was found at the scene.
No information was immediately available about what led the officer to fire.
The officer was not injured, Habeebullah said.
She briefed the media in an appearance that was posted on video on the police Twitter feed.
It was not immediately clear how many shots were fired by the officer, and the officer was not immediately identified.
The 300 block of Anacostia Road SE is a residential street that includes three-story brick apartment houses.
It is in the Fort Dupont area, east of the Anacostia River and the Anacostia Freeway.