The District government and the police officers union have reached a tentative agreement on a new contract that would provide a 9 percent salary increase over the next three years and a financial incentive to keep officers on the force longer.

The accord would offer a 5 percent pay raise to officers eligible for retirement, amounting to about $1,800 for every year 20-year veterans delay retiring. Two out of three city police officers are eligible to retire in the next five years.

The incentive to delay retirement is the city's first attempt to stem the imminent loss of two-thirds of the 3,880-member force. According to department figures, 2,351 veterans hired during the expansion period of the late 1960s will be able to retire by 1992.

"This is a small, small step in the right direction . . . the first time they've tried to stem the exit of these people," said Gary W. Hankins, chairman of the labor committee of the Fraternal Order of Police. "If it's not enough, we'll know by the summer of 1990 as we begin the negotiations for the next contracts."

The tentative agreement, the first of six major city workers' pay contracts to be hammered out this year, must be approved by the D.C. Council and ratified by the members of the Fraternal Order of Police before becoming final. The union is scheduled to vote Tuesday.

The agreement also would strengthen officers' dental, optical and legal benefits, award compensation time at time-and-a-half rather than the current straight time, safeguard seniority and hasten pay for temporary duty.

It also would assure that officers accused of using deadly force would be returned to full duty immediately after being cleared by in-house investigators rather than having to wait, in some cases nearly two years, for completion of a grand jury investigation.

An early accord with the Fraternal Order of Police was seen by both sides as an accomplishment after a bitter and protracted arbitration that led to the last contract.

City law prohibits public employes from striking, and unsettled contract issues are subject to arbitration. During the last round of negotiations, the police union won larger raises by going to arbitration.

Mark Levitt, director of the D.C. Office of Labor Relations and Collective Bargaining, and Hankins agreed that negotiations, which began July 15, were more arduous but less acrimonious than arbitration. Negotiators worked through the night Friday and into Saturday morning before reaching the tentative accord that city officials made public yesterday.

"The agreement is fiscally responsible and allows the District to be competitive for new recruits and retain experienced police officers," Mayor Marion Barry said in a prepared statement issued by his office. "I hope this agreement will provide impetus for prompt and mutually satisfactory resolution of other citywide negotiations."

The 3-percent-a-year pay increase was negotiated for the 3,600 officers through the rank of sergeant represented by the union. City officials said they hoped it would establish an acceptable pattern for the city's 18,000 blue- and white-collar workers whose contracts are expiring.

"I hope this sets the ball rolling for the other unions," said Levitt. "We like to look for pattern settlements. Generally we're looking for 3 percent settlements, more or less."

But Hankins said that the 9 percent pay increase over three years falls far short of the 30 percent increase he estimated would be needed to place D.C. officers on a "buying power" par with their counterparts in suburban jurisdictions where housing costs are lower.

Still, the pact overall represents a "good compromise," Hankins said.

"It makes their working environment better, provides them more rights in the areas of discipline, transfer, seniority and temporary details and kind of holds the line on pay," he said.