More than 6,000 armed robberies have been committed in the District of Columbia this year, almost twice as many as in 1979 and 25 percent more than the 4,765 reported last year, police said yesterday.

Small stores and shops, financial institutions and delivery trucks are being held up at an increasing rate as the year's end approaches, police said. There were 27 robberies and thefts on Sunday, 48 on Monday and 53 on Tuesday. In one of them yesterday, a 20-year-old Southeast Washington man was shot and killed by a police officer who interrupted a street robbery shortly after midnight.

Police said one of two "casual clothes" officers patrolling the unit block of O Street NE watched as two suspects seized a pedestrian and one of the men pointed a gun at his head. The armed man turned and fired at one officer, Gregory W. Rooney, as he approached with his gun drawn and shouted that he was a policeman.

The would-be robber's gun misfired. Rooney fired three shots, one of them striking the man, identified by police as Claude Marshall, of 4244 4th St. SE. He was pronounced dead at D.C. General Hospital.

The second suspect, a 16-year-old youth, fled, but was arrested by Rooney's partner, Mark A. Copanzzi. The youth was charged with armed robbery.

Rooney, who has been on the force for three years, was placed on administrative leave with pay, pending a routine grand jury investigation of the shooting, police said.

Police Chief Maurice T. Turner said his department has watched the rising number of armed robberies and thefts with frustration, but has stepped up efforts to control both types of crimes. They are the city's two most frequently committed felonies.

Tuesday's list of robberies included holdups at an insurance agency at 30 Kennedy St. NW, a Wisconsin Avenue dress shop, a Georgia Avenue tavern, a beauty salon and the Riggs National Bank branch at 4245 Wisconsin Ave. NW, the 102nd robbery of a financial institution in the city this year.

In addition, two armed men held up a liquor delivery truck at 10th and H streets NW. Police said the robbers tied up and blindfolded the driver and two assistants, placed them in the back of the Foreman Brothers Co. truck and drove to an unidentified location where they unloaded $10,000 to $15,000 worth of liquor. The robbers later abandoned the truck in Palmer Park, where the three men were freed after passersby heard their cries for help.

"It's frustrating, to the point that we have done everything we possibly can to try to stem this rising tide of robberies and larcenies," Turner said in an interview yesterday.

Among various tactics, the chief said detectives disguised as elderly men and women are being placed on the streets as decoys in areas where old people frequently are preyed on by robbers.