Handgun-related crime in the District of Columbia has decreased significantly in the last three years, largely because of the city's strict gun registration law that went into effect in early 1977, according to a study released today by the U.S. Conference of Mayors.

The findings were immediately challenged by both the D.C. police and the National Rifle Association, the national gun lobby.

The NRA repeated its long-held contention that gun control laws are systematically ignored or circumvented by society's criminal element.Police officials said that any reduction in gun-related crimes cannot clearly be attributed to gun registration but may have other causes including new law enforcement tactics and programs and long-term cyclical reductions in overall crime.

During the period studied, for example, the U.S. Attorney's office here began a program aimed at thwarting repeat offenders and D.C. police launched high visibility, concentrated crime-fighting operations in high crime areas. D.C. Assistant Police Chief Maurice Turner said these factors could have been just as responsible for any reduction in gun-related crimes.

The 33-page Conference of Mayors study of District crime statistics shows there was a 22.5 percent reduction in reported handgun robberies during the three-year period since gun registration compared to the three-year period prior to registration.

Similarly, it showed a 26.2 percent reduction in homicides by handgun and a 10.5 percent drop in assaults-by-handgun.

"This statistical analysis of firearm rates clearly shows that the Washington, D.C. gun registration law has had a significant impact on reducing handgun crime and should serve as a model for governmental jurisdictions throughout the country," said John Gunther, executive director of the mayors conference, in a statement released with the study.

The city implemented the strict and controversial law, which theoretically froze the number of handguns in the city, in February 1977. It required registration of all currently owned handguns, rifles and shotguns and banning the sale or possession in the future of any additional handguns by private citizens in the District.

While the Conference of Mayors study shows a general reduction in gun-related crimes in the three-year period since registration, critics note that the deadline actually started before registration was implemented and that all crime -- both gun-related crime and crime in general -- started to increase again in 1979, and is continuing upward today.

Armed robberies have increased 19 percent so far this year over the same period last year and more than doubled in the month of June, police reported recently. Police attribute the upswing to the warmer weather, idle youths out of school and the general downturn of the area's economy.

The Conference of Mayors study acknowledges the downward trend in gun-related crimes started more than a year before gun registration but contends the decline intensified after implementation of the law. The study also acknowledges crime, especially handgun robberies, began increasing again in 1979, but said this reflects, "a national trend" unassociated with gun registration.

The incidence of robbery -- the offense most often feared by citizens and the one viewed by police as the weathervane of general criminal activity in the city -- decreased from 1,141 per 100,000 city residents in 1974 to 940 in 1978, according to the study. In 1979, the rate rose to 1,043.

Within this general pattern, the subcategory of robberies with handguns for the same period shows a general decline from 508 in 1974 to 356 in 1978, before going up to 372 in 1979.

These figures show, the conference study says, that handgun robberies decreased after gun registration in early 1977 much more steeply than robberies as a whole and increased in 1979 more slowly than robberies as a whole -- all suggesting that gun registration is responsible.

Asked how he concluded there is a cause-and-effect relationship between the crime rates and gun registration, Robert Nicholson, a coauthor of the conference study, said, "with a leap of hope or faith."

With the close coincidence of gun registration and a decline in gun-related crimes, he said, "One just has to sort of assume the law had a positive impact."

He said other possible influences on the crime rate, such as unemployment, reduced per capita income and other socioeconomic pressures, are difficult to quantify and "factor in" to such a study.

"We just had to assume that economic factors did not have an impact," he said.

National Rifle Association spokesman John Adkins said the conference study "appears to be . . . a lot of rhetoric and manipulation of statistics."

There have been declines and increases in handgun-related offenses both before and after gun registration, he said, making it impossible to conclude anything about the law's effectiveness.

Besides, Adkins said, the habitual criminal "is not going to bother complying" with gun registration. "If he wants to commit a robbery or a rape or a homicide and he thinks he needs a gun, he'll get a gun."

Assistant police chief Turner, head of the department's field operations, noted that persons can still easily obtain handguns in the neighboring Maryland and Virginia suburbs.

He said less than 1 percent of all firearms confiscated each year by police are registered here or elsewhere, "so somehow or other, the illegal guns are still getting in here."

Police department figures show the number of confiscated firearms (mostly handguns) has dropped from more than 3,000 a year in the period before gun registration to about 2,600 a year since registration.

Turner said, however, that that appears to go "hand-in-hand with the general reduction in crime since 1976" and cannot be related to city gun registration.