Mark Anthony Watson, accused of murdering D.C. police officer Donald G. Luning last Sept. 14, pinned the policeman to the floor and shot him with the officer's own gun, a prosecutor told the jury during opening arguments at Watson's trial yesterday.

"It's not worth it, is it?" Assistant U.S. Attorney Harold L. Cushenberry quoted the 31-year-old officer as saying just before Watson allegedly pulled the trigger of Luning's revolver, fatally wounding him in the chest.

Watson's attorney said in court yesterday that Watson had shot Luning but contended that he did so in self-defense and that Luning had failed to identify himself as a police officer.

A 10-year veteran of the police force, Luning died 1 1/2 hours later at Washington Hospital Center. He had lived in Waldorf, Md., with his wife and two children, who are 2 and 3 years old.

In his opening statement, Cushenberry told the jury that Luning and his partner stopped Watson and a passenger because they thought the car Watson was driving was stolen. The prosecutor said that Watson, 19, and a second occupant fled.

Luning, a tactical officer in plain clothes at the time, followed Watson to an apartment at 3694 Hayes St. NE, where Watson had burst in on strangers, Cushenberry said.

Cushenberry said that Watson was sitting at the dining room table, where he had made a telephone call.

Drawing his gun, Luning told Watson, "You're under arrest," Cushenberry said. "For what?" Watson replied. "You're under arrest," Luning repeated. Again, Watson asked, "For what?"

"Get up or I'll blow your head off," the prosecutor quoted Luning as saying. When Watson got up from the table, Luning sheathed his revolver and prepared to place handcuffs on the suspect.

Cushenberry said Watson grabbed Luning, and the two men struggled. When the gun fell out of the officer's holster, Watson grabbed it.

"The last thing they the occupants of the apartment saw was a .38 at Luning's chest," Cushenberry said. He said the witnesses heard Luning's last words and a single gun shot. Watson surrendered at the scene after a long confrontation with dozens of officers, the prosecutor said.

Public Defender Stephanie Duncan-Peters, seeking a not guilty verdict for Watson, who is charged with first-degree murder, told the jury Watson stole the car and killed Luning but acted in self-defense.

"Luning wore blue jeans," Duncan-Peters said, claiming that he did not identify himself as a police officer. "Watson tried to avoid trouble with that man," whom she described as acting "crazy" and "trying to kill" Watson.

"To save himself, he fired the gun," she said.