The number of serious crimes reported in Alexandria increased in 1985 for the first time in six years, police officials announced yesterday.

The 7,670 serious crimes -- homicide, rape, robbery, aggravated assault, burglary, larceny and motor vehicle theft -- represented a 2 percent increase over 1984 figures, according to statistics released by the Department of Public Safety.

"There are a lot of ways to look at the figures," said Charles T. Strobel, public safety director. "But the overall change in most categories is not that significant -- and it's pretty much in line with what other areas in the region are reporting."

Strobel said the rise in crime could be a result of a recent population increase or the steadily rising number of people who work in the city and live elsewhere.

The biggest jump was in aggravated assault, up 7 percent from 1984, although police officials pointed out that the figure increased by only 22 incidents in 1985. Rape fell by 12 percent from 59 to 52 incidents.

Motor vehicle thefts, which long have plagued Alexandria, increased by almost 6 percent to 668 last year, although the rate of increase was smaller than in several recent years.

Police said that because the city has a large number of open parking lots and is located at the junction of several major highways, it has always been an area leader in auto theft.

"This is something that we are seeing increasing around the region," said Strobel, "and it is something that disturbs us very greatly. But it is difficult to stop."

The total number of arrests in the city climbed by 11.5 percent to 31,000 last year.

The rate at which the city was able to resolve its reported serious crimes -- by arrests or other means -- remained relatively stable at about 21 percent.