The trial of a man charged with murdering a Prince George's County policeman began yesterday in Upper Marlboro, with the county's top prosecutor, in a rare courtroom appearance, telling jurors that Harlow Bryant Sails "calmly" shot Officer Raymond Hubbard while Hubbard was lying seriously injured in a corridor at Iverson Mall.

State's Attorney Arthur A. Marshall Jr., who generally restricts his role as prosecutor to alleged police killers, is seeking the death penalty for Sails, 20, of Forestville, who has pleaded innocent of the Feb. 8 killing that occurred when Hubbard, 28, attempted to stop several men men fleeing from a jewelry store robbery.

Ronald Wayne Drake, 24, was found guilty last August for his role in the armed robbery and murder, and was sentenced to life. Two other men, Terrance A. Harris, 21, and Sails' older brother, Horatio Kermit Sails, 24, subsequently pleaded guilty and were sentenced to life in prison. Another suspect is still at large.

But it was Harlow Sails who came from behind a kiosk on the mall's second level, brandishing a 9 mm Browning automatic and firing at the officer "probably about eight times," Marshall said yesterday. Five of the shots hit Hubbard, Marshall said, the final one killing him.

Hubbard, who had come to the mall to shop, "was severely disabled and collapsed on the floor, moaning," Marshall said. "With this, the defendant came over and calmly leaned over Officer Hubbard, and fired a fifth shot, into his spinal column and heart."

Marshall said Sails then "calmly leaned over Officer Hubbard again," took his service revolver from him, and left the mall, shooting a man who tried to trip him, two times in the leg. Outside the mall, Marshall said, the defendant "very calmly and peacefully" ordered a man at gunpoint to get out of his car. Sails then drove away in that car.

Marshall said the car was later found burned, and that prosecution witnesses, expected to testify throughout the week, would show that items stolen from the jewelry store were later found in Sails' residence, as was Harris' driver's license.

Defense attorney Michael L. Gallavan, who declined to make an opening statement to the jury, spent much time yesterday questioning the validity of an identification made by a witness who said she saw three men walking toward the jewelry store minutes before the robbery.