An off-duty Prince George's County police officer and an employe of a convenience store in Greenbelt were both shot and fatally wounded late Saturday night by robbers who took $25,000 in a holdup at the store, county police said.

The officer, Carlton X. Fletcher, 31, who worked at the store part time, was struck in the neck by a shotgun blast apparently fired shortly after he arrived in a police cruiser to escort the employe to a nearby bank to deposit the night's receipts.

Police said Fletcher was not in uniform when he arrived at the store, the Goddard Mini-Mart, a combination convenience store and filling station on Greenbelt Road, a few blocks from the gate of the Goddard space flight center.

Police said, however, he was operating a police cruiser under what was described as standard procedure for officers who work at part-time jobs.

The store clerk was identified by police as Minh Phamdo, 20, of Seabrook.

Fletcher died at Prince George's General Hospital at 12:51 a.m. yesterday. The store employe died there at 4:10 p.m.

Police said they are looking for two suspects in connection with the shootings.

Although details of the incident remained unclear last night as police continued to seek witnesses, accounts provided by authorities and persons in the area indicate that Fletcher, a seven-year police veteran was seated in his vruiser in the store parking lot about 11:45 p.m. waiting for Phamdo.

Within moments, according to these accounts, both the officer and the store employe were struck by shotgun blasts.The officer was hit in the left side of the neck by a blast fired either as he sat in his car or as he was just emerging from it.

At least one of the windows on the driver's side of the cruiser was shattered. The windshield was splattered with blood, a witness said, and the inside of the car was also bloody.

Police said Fletcher never removed his service revolver from its holster.

The store employe was hit in the chest by a blast apparently fired immediately after the officer was shot.

The robbers fled with a sum estimated at about $25,000 by a representative of ARCO, which owns both the store and the filling station.

Police said Fletcher was taken to the hospital by ambulance. Phamdo, whose chest wound was described as massive, arrived at the hospital by Maryland State Police police helicopter.

Fletcher was described by his superior, Sgt. Anthony Amoia of the Beltsville substation, as "a quiet individual . . . conscientious and very family oriented. He was looking forward to having a second child. That's why he was working part time. He talked about buying a house."

Fletcher's daughter is three years old and his wife is six months pregnant, Amoia said. According to the sergeant, Fletcher had been moonlighting on the Greenbelt job for approximately a year.

"He'd always watch what you were doing and make sure no one ran over your hoses. He was a thoughtful man," said ARCO driver Ed Jackson, who said he often drank coffee with the officer after his night gasoline delivery. "Normally I would've been here at that hour, but I had to go to a graduation," he said.

Phamdo, a Vietnamese engineering student who was working nights to help pay his tuition at the University of Maryland, was described by the store manager as "smart, a hard worker" who had "a good attitude. He was always smiling."

Several ARCO representatives drove to the scene from Philadelphia yesterday. They said the killings were the first at any of the company's franchises in seven years.

Fletcher is the second off-duty Prince George's County officer to be killed in just over a year. In early 1982, Officer Raymond Hubbard was killed during a shootout after he interrupted a jewelry store robbery in Iverson Mall.

Two persons were stopped near the Baltimore Washington Parkway early yesterday by police investigating the shootings at the service station, but they were released after questioning.

In explaining the difficulty of providing details about the shootings, a police spokesman said last night that no customers were believed in the convenience store portion of the Mini-Mart at the time of the incident.

The spokesman asked that anyone in the vicinity of the site call county police homicide investigators at 967-5600.