In a heavily-guarded courtroom, a former District drug dealer told a Prince George's County Circuit Court jury yesterday how a convicted heroin dealer shot a 24-year-old Northeast man suspected of stealing heroin from another figure in the city's drug underworld.

In describing the June 1981 "execution-style" slaying in a Prince George's County park, Ronnie Wilder, serving a 21-year sentence for bank robbery in an Alabama penitentiary, provided a glimpse into the vicious world of Washington's thriving heroin trade.

Because of a threat against Wilder's life, security is tight at the trial of Charles Watkins, 20, of Southeast Washington, who is charged with the murder of alleged drug dealer Eddie Harris. Witnesses are guarded in the courtroom by members of the police department SWAT team, in addition to sheriff's deputies. Defense attorney William J. Parker Jr., who always wears a flower in his lapel, also wore a bulletproof vest in the courtroom on Monday, and a deputy sheriff used a metal detector to screen spectators.

Assistant State's Attorney David Simpson said "there was some fear" that the names of other prominent Washington drug dealers would come up in the trial and an attack against Watkins or certain witnesses "would be one way of shutting them up" before they implicated others.

Wilder testified yesterday that on the night of June 23, 1981, he drove to a park on Shady Glen Terrace near Capitol Heights with Watkins, Harris and Darren Brigman, ostentsibly to test some heroin. But once there, Wilder testified, Watkins drew a .38-caliber gun on Harris.

" 'This is it,' " Wilder said Watkins told Harris. "Eddie raised up his hands like he was surprised. A shot went off that hit him in the chest."

Standing up in the witness box and demonstrating to the jury, he said that Watkins then stood over Harris and shot him four more times at point-blank range. The bullets hit Harris in the face and head.

"It went pow, pow-pow-pow, click," Wilder said.

Brigman also testified yesterday that Watkins killed Harris.

Cpl. John San Felice of the Prince George's police testified that Watkins told him that a contract had been put out on Harris' life because Harris supposedly stole heroin and diamonds from another drug dealer. Felice also said Watkins first told authorities that Brigman killed Harris, then maintained later that Wilder committed the murder.

On cross-examination, Parker suggested that Wilder had implicated Watkins so that Wilder would not be charged with the killing. Parker noted that police originally had charged Brigman with the murder and that Wilder and Brigman have been granted immunity from prosecution in exchange for their testimony against Watkins.