Serious crime in Alexandria declined by 4.6 percent in the first half of this year compared to the same period a year ago, city police officials said yesterday.

Reported rapes, robberies, burglaries, larceny and motor vehicle thefts all declined, authorities said. The largest decrease was in rapes, which fell from 28 to 20, a drop of 28 percent.

Police statistics showed a 6 percent decline in both larceny and motor vehicle thefts.

Aggravated assaults increased to 167 from 138 cases in the same period last year, a 21 percent increase. Police said many aggravated assaults involve individuals who know each other.

"The greatest number are within the family unit. It's very difficult to gear up and deploy our forces there," said Alexandria Public Safety Director Charles T. Strobel.

Strobel said he attributes the drop in crime to improved community outreach programs and to the professionalism of the police department, which has been working under the strain of public criticism over the past year.

"Quite frankly, I credit the men and women of the Department of Public Safety. I say this especially regarding the issues the police deparment and I have been facing in the past five or six months," Strobel said in reference to legal entanglements stemming from his decisions to transfer several police officers.

In a civil suit last week, a federal jury found that the public safety director had punitively transferred two employes, and awarded them $15,000.

Crime in Alexandria has decreased steadily since 1979, Strobel said.

The number of serious crime offenses fell from 10,021 in 1979 to 7,521 in 1984, a decline of about 25 percent, according to police records.

Fairfax and Montgomery counties, the area's two most affluent jurisdictions, recently reported increases in serious crimes for the first six months of this year.

Law enforcement officials there blamed the rise on increased population and development growth.

In Arlington, police have reported a 2.2 percent increase in serious crime.

Crimes fell in the District by 10 percent, officials said.

Alexandria police have attempted to combat crime by keeping in close contact with neighborhood civic associations.

A member of the police department is present at most of the monthly neighborhood meetings, Strobel said.

The department has also stepped up patrols in areas where drug traffic, public drunkeness and loitering have been chronic problems, notably an inner city neighborhood bordered by First Street, Cameron Street, Columbus Street and the Richmond, Fredericksburg and Potomac Railroad yards.

The number of crime cases closed by arrests or other means rose from 705 in the first half of 1984 to 765 in the first half of this year.

Strobel said he considers this significant because case closures have increased as crime decreased. Normally, the rate of cases solved parallels the volume of crime, Strobel said.