The recently announced resignation of Alexandria's only top-ranking black city employe has prompted charges of discrimination from local black leaders and vehement denial of those charges from city officials.

Gilbert H. Bradley, 43, controversial director of the city's Human Services Department and the only black department head in Alexandria government, announced last week that he will resign from the post he has held almost five years to take a job with a private investment firm in Falls Church.

His announcement comes two months after a series of budget hearings during which he was sharply criticized by Democratic Vice Mayor James P. Moran for ineffective management and inadequate concern for the poor. The criticism stemmed in part from Bradley's failure to spend about $43,000 in city social service money, thereby forfeiting matching federal funds.

City officials had increasingly criticized Bradley, who acknowledged seeking work in the Reagan administration in 1980. Moran said yesterday that he had "discussed his lack of enthusiasm" for Bradley with City Manager Douglas Harman.

Bradley said he decided to quit more than a year ago and was prompted by a growing dissatisfaction with social service work, though he described the criticism he received as "different and unusual."

"Something's different when you don't see any other department head being raked over the coals in public for failure to spend $43,000," Bradley said.

"I thought he did a very good job," said Nelson Greene, a recently retired black councilman. "I think they council were a little tough on him. I've never known them to do that to another department head."

Green said blacks "always get a little different treatment from city hall." Whether the scarcity of blacks in high-level city positions "was by intent or attrition, I don't know. We'll find out when they appoint someone new."

Harman said the city will place a "high priority" on finding a minority person to fill Bradley's job, though he said it is difficult to attract qualified blacks to work in the city because they often are hired for higher- paying jobs in the federal government and the private sector.

Blacks make up about 22 percent of Alexandria's population and more than 50 percent of its public school population. Black leaders often have said they are underrepresented in city government. Alexandria has one black on its seven-member city council and two blacks on its nine-member school board.

There "is a seeming pattern of attack to discredit many of the black administrators and employes in the city," said Northern Virginia Urban League director George Lambert.

"That's absolute bull," Moran said yesterday of allegations of possible discrimination. "This had nothing to do with race whatsoever. Anyone who says so is giving a knee-jerk, asinine response. In fact, he Bradley probably would have been gone a lot sooner had he not been black.

"The thing that bothered me the most was that he kept telling us everything was copacetic, running smoothly, but I kept hearing nothing but horror stories from department workers. His performance was denigrating the very people he was supposed to be helping."

Bradley explained yesterday that the disputed $43,000 was an unforseen surplus he planned to save as a contingency fund during the last three months of the fiscal year.

"That money could have been put to good use in three months," Moran said. "It doesn't take three months for families to find food. Children can get very much abuse in three months."