Serious crime in Washington decreased by about 10 per cent last year compared to 1975, with a drop of more than 5,000 in the number of offenses reported in the first 11 months of last year, plus projections for the month of December, for which complete figures are not yet available.

However, based on preliminary indications, police say they expected December to be the 21st consecutive month in which the serious crime reported here has declined over the corresponding month in the previous year.

In 1975, the total number of serious crimes reported to police was 55,158. The corresponding figure for 1976 is expected to be 49,722, a decline of 5,436, police said.

Crimes listed in the serious category are homicide, rape, aggravated assault, auto theft, burglary, larceny and robbery. Police said the decline has been particularly sharp in the number of reported robberies, general regarded as a sensitive barometer of criminal activity.

There were 8,333 robberies of all types reported in the first 11 months of 1975, compared with 6,408 - almost 2,000 fewer - in the corresponding period of last year.

In addition to the 23 per cent decrease in all robberies, Capt. John C. Connor of the robbery squad said that armed robberies declined by the same percentage, from 4,979 in 1975 to 3,836 last year.

He called the decline the largest in his career as a detective.

Although the percentage figures vary, the number of crimes in all seven of the "serious" categories declined last year, according to Capt. Whitmore Britt, head of the police planning data bureau.

He released the following figures:

In previous interviews, D.C. police chief Maurice J. Cullinane has cited several factors as contributing to the reduction in crime.

These include increased foot patrols, some of them involving scooter-mounted officers who frequently park their vehicles to tour their assigned beats on foot.

Cullinane has also cited a recent crackdown by police and prosecutors on suspects arrested repeatedly for crimes, a continuing drive by police against narcotics traffic, existence of drug treatment centers and increased cooperation by local residents with the police.

He has also noted that the reduction in crime has occurred despite the decline in the size of the force from 5,100 officers in 1969 to the current level of 4,300.