Major reported crime decreased 5 percent in August compared to August a year ago, the first monthly decline in almost 1 1/2 years, D.C. police said yesterday.

Major offenses -- homicide, rape, robbery, burglary, car theft, larceny and aggravated assault -- totaled 6,233 in August, 297 fewer than the 6,530 reported for the same month last year.

But crime for the first eight months of this year as a whole has continued upward, running about 8 percent above the same period last year, according to police statistics.

Assistant Police Chief Marty M. Tapscott said, however, that the 8 percent figure is down from 13 percent reported for the first six months of the year compared to last year.

Tapscott said he expects "this extremely positive trend to continue." He attributed the improved figures to increased citzen help in assisting police and new anticrime efforts by the city.

Specifically, he said, the IN FACT program, in which city government employes, taxi drivers and utility workers with two-way radios in their vehicles are encouraged to report suspicious persons to police, resulted in 247 calls and 80 arrests, including a homicide suspect, between June and August.

Tapscott also said there are now 9,600 households enrolled in the police neighborhood watch program, in which residents keep an eye out for one another's homes, monitor streets for unusual activity and call police when they see suspicious cars or individuals. He credited the program in part for a reduction in reported burglaries in August, down from 1,663 in August 1980 to 1,439 this year.

A special police narcotics task force, which has arrested more than 600 suspected drug users and sellers since a major crackdown was started in mid-August, has taken some of the drug traffic off city streets "at least temporarily," Tapscott said.

Police say some drug users contribute to the increase in crime as they rob and burglarize to support their habits.

According to statistics for the first eight months of this year, homicides are up 11 percent from 126 for the first eight months a year ago to 139 for the same period this year. Robberies jumped 23 percent from 5,165 to 6,342 and burglaries increased 9 percent from 10,054 to 10,964.

The number of aggravated assaults rose 8 percent from 2,153 to 2,317, while larcenies increased 5 percent from 20,158 to 21,037. Car thefts increased 4 percent from 2,325 to 2,408, and rape decreased 4 percent from 288 to 275.