A 31-year-old Prince George's County police officer was shot in the neck and fatally wounded late last night after he arrived at a service station in the Greenbelt area while a holdup apparently was under way.

A male employe of the Goddard Mini-Mart, a combination convenience store and gasoline station just outside the Goddard Space Flight Center, was also shot during the incident and was reported in critical condition early this morning.

Witnesses said the officer was struck by a shotgun blast immediately after opening the door of his cruiser at the service station.

A police spokesman said the officer was off duty, but had use of the cruiser under a special program.

Two men were stopped on the Greenbelt Road (Rte. 193) overpass at the Baltimore-Washington Parkway, about two miles from the shooting scene, early this morning for questioning in connection with the slaying, county police said.

The officer, who was not immediately identified, was taken to Prince George's General Hospital where he died shortly after the 11:45 p.m. incident.

According to the accounts of witnesses and store employees, two men were holding up the Mini-Mart, located at 8711 Greenbelt Rd., shortly before midnight when the officer drove up to the area near the gasoline pumps.

The robbers had already taken an undetermined amount of money, and according to preliminary accounts, appeared to be leaving, when the officer opened the door of his county police car.

Witnesses, who included motorists whose vehicles were parked at the pumps, said one of the robbers fired a shotgun as the officer emerged from his car.

The officer, a District resident, was struck on the left side of his neck.

Glass on the driver's side of the police car was shattered, and the inside of the vehicle was spattered with blood, witnesses said.

The employe, whose name was not released, was taken by helicopter to Prince George's General Hospital.

A hospital official said the man was shot at least once in the chest and was in surgery early today.

A police spokesman was unable to provide additional information about the shootings.

Shortly after the incident, the scene was cordoned off and a description of a foreign car bearing out-of-state license plates was broadcast over police radios.

The two men were still being questioned at 2 a.m. by Prince George's County police.

Greenbelt police, assisted by U.S. Park Police, were involved in stopping the men.