Serious crime has decreased dramatically throughout the Washington metropolitan area this year compared to last year, a drop that is also reflected across the country.

D.C. police reported yesterday that serious crime decreased 11 percent in the first six months of this year compared to the same period last year. Police Chief Maurice T. Turner Jr. attributed the decline to anticrime projects undertaken by neighborhood residents and to police programs that focus on arresting repeat offenders.

In Washington, as in the other jurisdictions, the designation of "serious crime" is given to homicide, rape, robbery, aggravated assault, burglary, larceny, and car theft.

Prince George's County police said serious crime there went down 14.6 percent in the period from January to June compared to the same months last year, and Montgomery County police reported a 16.2 percent drop.

In Virginia, Fairfax County police said serious crime decreased 9.3 percent in that period, Arlington County police reported an 8 percent drop, and Alexandria said serious crime was down 15 percent.

Many police departments all over the country, including those in urban, suburban and rural areas, have reported similar decreases in serious crime so far this year compared to last year, criminology specialists said. New York City, for example, has reported a 9.4 percent decrease for the first six months of the year, while San Francisco has reported a 6.1 percent decline.

One main reason for the decrease, which experts said began about two years ago, is a drop in the number of people under 25 years of age, the group most likely to commit serious crimes, said Robert Angrisani, spokesman for the International Association of Chiefs of Police, a private research organization.

Other reasons cited by police departments in this area and around the country are the fact that judges are handing out stiffer sentences, and the kinds of neighborhood-watch programs cited by Turner, Angrisani said.

"By God, they work, and they work beautifully," Angrisani said of the community anticrime projects, in which neighbors agree to watch out for crime and report it. "They are a very, very important factor in this decrease."

In Washington, the number of homicides decreased from 102 in the first six months of 1982 to 97 so far this year. Burglaries decreased 19 percent, from 7,496 to 6,052, and larcenies from cars went down 19 percent, from 8,590 to 6,975.

The total number of serious crimes reported decreased 11 percent from 32,181 to 28,506. Figures show a 14-month trend toward fewer serious crimes, Turner said.

The D.C. 2nd Police District, comprising Georgetown and upper Northwest, recorded a decrease of 21 percent in the number of serious crimes reported, the largest decrease of any of the city's seven districts, each of which reported a drop.

Turner said some credit for the decrease should go to the department's Repeat Offenders Project, which focuses on arresting the most active criminals, and the Narcotics Task Force, which focuses on drug dealers and buyers.

In Prince George's County, the number of homicides decreased 21.9 percent, from 32 to 25, for the first six months of 1983 compared to the same period last year. The number of rapes reported dropped 13.9 percent, from 180 to 155, and burglaries dropped 25.1 percent, from 5,495 to 4,116.

Police Chief John McHale of Prince George's County said neighborhood-watch programs and added manpower are partly responsible for the drop. Crime statistics have been declining in Prince George's County for the last 22 months, at the same time 100 officers were been added, McHale said.

In Montgomery County, aggravated assaults declined 22 percent, from 415 to 323; robberies decreased 22 percent, from 354 to 274, and burglaries went down 15.4 percent, from 2,843 to 2,404. All five of the county's police districts showed decreases, and Bethesda led the county with a 22.8 percent drop.

Sgt. Harry Geehreng, a spokesman for the Montgomery County police, said tougher prison sentences play a role in the decrease.

Police officials in Fairfax, Arlington and Alexandria echoed the view that anticrime groups helped cause the decrease.

After decades of relatively low incidence of serious crime, statistics ballooned in the mid-1960s and 1970s, criminologists said, in part reflecting the greater numbers of young people from the postwar baby boom.

Criminologists predicted the current decrease several years ago because of the fewer number of young people, said James Fyfe, an associate professor of justice at American University. He said experts also expect crime statistics to increase around 1990 because of a more recent baby boom.

The improved economy is another factor in the decrease in crimes against property, Angrisani said.