On an average Saturday night in Prince George's County, about 80 police officers are walking their beats or patrolling the streets in their cruisers.

At the same time, an equal number of county officers are standing guard in liquor stores, fast-food outlets and shopping centers. Although most of these officers also are wearing official police uniforms, carrying department revolvers and arriving to work in police cars, they are not working for the county.

They are, as one officer put, "Prince George's second police force."

This second force consists of off-duty officers who moonlight as private security guards at business establishments in the county. Although the department will not disclose moonlighting statistics, police sources indicate that an overwhelming majority of the county's 850 officers -- perhaps 85 percent of them -- work part-time as security guards.

Unlike other Washington area jurisdictions, police in Prince George's are routinely permitted to wear their uniforms while working in off-duty jobs like security guard. Some other localities forbid off-duty officers to carry their guns during off-duty employment -- one notable exception being the District of Columbia where officers must carry their guns off duty but are forbidden to take security guard jobs.

The wisdom of the Prince George's policy has been questioned by some since the shooting death last weekend of Antonio M. Kelsey, a county officer who was killed in an incident that involved his off-duty work as a security guard at Cox's liquor store in Landover.

"It raises the question of who's liable," said Len Colodny, former chairman of the law enforcement practices panel of the county's human relations commission. "After all, the officer is wearing the identification of Prince George's but he's being paid by the owner of the establishment, like McDonald's. The question is under whose orders is he acting -- the chief of police or the owner of McDonald's."

Police department officials say they are constantly debating the moonlighting policy. Some have recommended from wearing their uniforms while working off-duty. But that idea has been opposed quite vigorously by the rank-and-file police and the businessmen who employ them.

"If I didn't think it was protective, I wouldn't hire the officers," said one Silver Hill liquor store owner. "We get hit the most. We had those Rent-a-cop Services, but they're nothing but trouble. They have no authority and everyone knows that."

Added one officer who has his own off-duty security agency: "If our men were not allowed to work part-time, the county would have to double the size of its police department."

Internal department documents show that for a given weekend last month, 195 officers worked as security guards in more than 100 locations. These officers worked a total of 1,000 hours for the weekend, and 60 percent of them wore their uniforms. Police sources peg the hourly pay for an off-duty officer at approximately $10 an hour, so the off-duty officers brought home about $10,000 for the weekend.

According to the internal time sheets, officers worked as short as two-hour stints and as long as 16 hours at locations such as McDonald's, 7-Elevens, various liquor stores, apartment houses, Drug Fair, Sheehy Ford, People's Security Bank, Woolco, Penn Mar, Jack-in-the-Box, and Club Le Baron.

Most of the officers who were interviewed said the only reason they work the extra jobs is to supplement their income. "It's important for me so I can feed my kids," said Cpl. Harold Ruslander, who sometimes walks an off-duty beat at Landover shopping center. "The regular pay is not that high and I want to maintain a normal standard of living."

While on duty, county officers work four-day, ten-hour shifts. The department regulations say an officer may have up to 16 hours of part-time off-duty work during those days, and an unlimited amount of such work on their days off.

"It does create a question of what kind of product do you get," said one county officer. "After working 10 hours for us, how alert is he (on a part-time security job)?"

The "second police force" generally costs twice the amount a private security firm will charge. "You're going to pay a lot for a police officer -- it's a hired gun with the power to arrest," said one liquor store owner. "But they command a considerable amount of respect, particularly in tough neighborhoods."

The county also permits officers to drive their police vehicles to and from the off-duty jobs. "That cruiser in the parking lot is certainly a deterrent," said John McGraw, of Shope-Rite Liquors, in Takoma Park.

The president of the county police union, Laney Hester, considers the off-duty opportunities "economic necessity," for the officers. Moreover, he says, with the county's TRIM budget-cutting measure in effect, moonlighting provides an economic way to increase police presence at no cost to the county.

"If our police presence is 100 officers working, and we have another 100 guys working off-duty as security guards in uniform, we've doubled the police presence and not paid them a thing," and Hester. "We've relied on the private sector to do that."

Most county businessmen said they do not mind paying extra for protection. "We know these are hard times," said Edith Chialastri, manager of Cox's Liquors, where Kelsey was working on the night he was killed. "And we like to help the police while they are helping us."