FOR DECADES, charges of police brutality have dogged the Prince George's County force. Tales of officers' abuses -- real and alleged -- are legion. So it's no surprise that a recent poll commissioned by the police chief reveals that one-third of the county's residents think the police use excessive force. And black residents particularly -- 48 percent -- think the same about a department whose racial composition, while changing, still fails to reflect that of the county.

But the poll also revealed something else that may come as a surprise: a large majority of county residents, black and white, think the police are a credit to the community. A contradiction? Not necessarily.

The perception of brutality stems from incidents such as the most recently publicized one, in which a white officer was found guilty of using excessive force when he struck a black youth with his nightstick while attempting to arrest him for littering outside a convenience store.

But the attention given to cases such as this tends to obscure other facts one should keep in mind when evaluating the work of the P.G. police. One is that despite the number of charges made against the P.G. police -- 44 in 1986 -- most are not upheld following an investigation by a hearing board, the proceedings of which are open to the public.

The second is that the county has, with the exception of the District, the highest crime rate in the region; police work there can be a dangerous business (there were 620 assaults against officers in the line of duty in 1986 -- eight times the number in Fairfax).

Finally, the police department has made great strides in recent years to clean up its act. County Executive Parris Glendening and Police Chief Michael Flaherty have emphasized repeatedly that they will not tolerate the use of excessive force. And the department has instituted hiring policies and procedures intended to improve its relationship with the community: one out of two new recruits is black (the force is now 26 percent black); officers accused of excessive force must seek counseling; all veteran officers are trained in "human relations"; and the chief meets regularly with members of the black community.

"The old P.G. department -- it's gone," says Capt. Jack San Felice, a police spokesman. But its image -- that of a department prone to use excessive force -- apparently persists. "We have to work hard to change that," says Capt. San Felice. From all indications, they are.