D.C. Police Chief Maurice Turner said yesterday that the District is continuing to have a decline in serious crime, but illegal drugs remain a major problem that is "eroding the quality of life in the city."

"The biggest single source of complaints in this office are drugs," said Turner, who added that "somewhere between 50 and 60 percent" of burglaries, robberies and larcenies committed in the city are drug-related.

Nonetheless, Turner said, more efficient police work and greater citizen involvement in fighting crime are a major reason that serious crime is decreasing.

"I don't think any chief of police would say there is an acceptable amount of crime. But I am confident . . . we are experiencing a decrease in crime. People are taking protective measures . . . and our effort is not going for naught," Turner said in an hour-long interview in his office yesterday.

Turner based his statement on department figures that show that the reported crime rate this year is running from 7 to 8 percent lower than last year. The decline in crime in the District mirrors a similar nationwide and area trend since 1982.

U.S. Attorney Joseph diGenova, in a separate interview yesterday, praised police and citizen efforts that have contributed to the decline in reported crime, but he said arrests, particularly for drug offenses, continue to rise. DiGenova said reported crime statistics, which are confined to such offenses as burglaries, murders and assaults, do not give the full extent of the crime problem because they do not include narcotics offenses, which are continuing to skyrocket in the District.

Drug indictments in Superior Court, for example, have increased 262 percent over four years ago, diGenova said.

"We have a drug problem in this city that is just frightening in its dimensions," diGenova said. "It cuts across cultural and economic lines."

Turner said if arrests continue to rise -- and he believes they will -- and overcrowded conditions persist at the D.C. Jail and in other city prison facilities, then he believes the city will need to build a new prison.

Mayor Marion Barry believes a new prison is not needed under present conditions, but will continue to monitor the situation and consider all "options and alternatives," his press secretary, Annette Samuels, said yesterday.

Among the other topics discussed by Turner were:

* Threats against the mayor and his family require almost full-time protection of Barry's wife Effi and their son. Turner said police guards drive Effi Barry to and from work. Recently, a man was committed to St. Elizabeths Hospital after threatening to show up at Barry's residence with a gun, Turner said.

* The police department is not involved in the federal investigation of allegations that Karen Johnson, a former city employe and convicted cocaine dealer, supplied cocaine to Barry -- allegations that Barry has denied. Turner said he has not discussed the case with Barry.

* The department has submitted the findings of an internal investigation into allegations of misuse of its $300,000 fund for undercover investigations to the U.S. attorney's office, which is investigating the matter. Turner said that Capt. Sammie Morrison, the department budget officer in charge of the fund, has been "officially reprimanded" in an unrelated incident for using a police department boat for recreation, but Turner said that has nothing to do with the fund investigation. He said he has complete confidence in Morrison.

Police union spokesman Gary Hankins criticized the department yesterday for following a "double standard" that results in relatively lenient penalties for ranking officers such as Morrison. Hankins said internal department policies have the effect of discouraging aggressive police work, which in part may explain the drop in reported crimes.