Serious crime in Alexandria dropped by more than 15 percent during the first half of the year, the sharpest decline in five years, the Alexandria Police Department said yesterday.

Police Chief Charles T. Strobel gave partial credit for the decline, which saw dropping rates in reported incidents of all major offenses except murder, to growing community involvement in crime prevention. "Anything we do in assisting in bringing the community together is very advantageous to us," he said.

He cited recent steps to organize citizens into Neighborhood Watch groups that monitor their streets and homes and report suspicious activity to the police.

He also credited the department's Crime Solvers program, which offers cash rewards for tips on criminals, and the Career Criminal Program run by the Commonwealth's Attorney office, which concentrates on repeat offenders.

Rapes showed the greatest decline, dropping 26 percent from 34 incidents in the first six months of 1982 to 25 in the first six months of 1983. Strobel said increased public education efforts by the city's women's commission may have helped make women more aware of how to avoid rape situations.

Burglaries were down 13 percent from 896 cases to 783. Motor vehicle theft dropped 3 percent to 229 cases and larceny was down 18 percent to 2,094 cases.

Robberies declined by 16 percent. Strobel said better monitoring of areas where robberies are frequently reported and vigorous prosecution of repeat offenders helped lead to the decline.

"We're taking more of them out of the system and certainly this affects our robbery rate," Strobel said.

Homicide was the only category to rise. Four homicides were recorded in the first six months of 1983, compared to two in that period in 1982.

Strobel said he meets with the Alexandria Commonwealth's Attorney's office once a week about taking a "hard-nosed" approach in prosecution of repeat offenders.

He said robbery is down partly because more Alexandrians reduce the opportunity for such incidents by being aware of their surroundings and traveling in pairs at night.