The number of reported armed robberies in the District of Columbia increased 48 percent in 1980 over 1979, according to preliminary D.C. police figures, the steepest annual increase in the last decade. Figures for January indicate the increase is continuing this year as well.

Robbery squad detectives said armed robberies totaled 4,677 in 1980, compared jwith 3,155 the previous year. Also, they said, armed robberies skyrocketed to 626 in January this year from 362 in January 1980.

Police officials give varying reasons for the steep climb, including unemployment and the general economic downturn in the last year, increased availability of heroin and other illicit drugs on the street, reduction in the police force and failure of the local courts to imprison significant numbers of habitual criminals.

Deputy Chief Alfonso Gibson, commander of detectives, voicing a frequent police complaint, said less then 2 percent of all suspects arrested and convicted wind up behind bars. "We arrest them," he said, "and then for some reason or another, when they get through the courts, they're back out on the street again."

Other police officials said the increase in armed robberies (and what imcomplete figures show is a somewhat less striking increase in burglaries) are not unique to Washington but are part of a national phenomenon. "Every major city in the county is showing a similar increase in particularly these two types of crime, robbery and burglary," said Police Chief Burtell M. Jefferson.

To combat the trend, he said, he has recently recruited 24 new officers, instituted new patrols, redesigned old ones and obtained authority to pay a limited number of officers to work overtime in high-crime areas.