Some top D.C. and U.S. law enforcement veterans have decided to retire during the last week of September to become eligible for maximum pension benefits available until then under a liberal retirement schedule.

The list includes two of the D.C. police department's four assistant chiefs, two inspectors, a captain and three lieutenants -- 26 men in all so far. Chief of Police Burtell M. Jefferson said the number could go as high as 50.

In addition, U.S. Park Police Chief Jerry L. Wells, several top officials of the D.C. fire department and an assistant director of the Secret Service also are retiring.

By retiring on or near Sept. 30 of every year, police and firemen and 1,500 federal law enforcement employes are able to include the annual pay increase awarded in October in their retirement benefits, police officials said. That can mean as much as $2,500 more a year to top-level retirees.

Assistant Chief of Police Robert L. Rabe, 51, is among top-ranking police officials who will take advantage of these retirement provisions. As expert in dealing with terrorism, Rabe successfully conducted critical negotiations for hostages during the 1977 takeover of several buildings here by Hanafi Muslims.

Also retiring is Assistant Chief John S. Hughes, 62, who has served 37 years on the force.

Wells, head of the 680-person U.S. Park Police force, said his decision was based primarily on "family considerations . . . my wife and my kids have been after me to stop. And even if I wait another 100 years, I'm not going to be making any more."

The special retirement system for D.C. and certain federal law enforcement personnel has stirred controversy on Capitol Hill and elsewhere because of its generous benefits, which are not available to other D.C. and federal employes. As a result, the system will be tightened in several areas next year.

Most of the controversy has focused on disability retirement benefits. However, the optional retirement system also offers advantages that will be cut back next year.

The system allows certain law enforcement personnel, for example, to retire after 20 years of service regardless of age. Most other city employees must be at least 50 years old.

For employes hired after this year, the system will require at least 25 years of service before full retirement benefits are available, according to a personnel official. Also starting in 1980, a "double-dipper law" will prevent the retirees from taking second-career jobs in the D.C. government, he said.

Many of these men, still in their early 40s or 50s, are looking forward to second careers, with their paychecks buttressed by retirement incomes of up to 80 percent of their current working pay.

Chief Rabe, for instance, said yesterday that he was retiring because he had an opportunity to form a consulting firm that will deal in his speciality -- crisis situations -- which include terrorist activities.

"We're losing a lot of experienced men," noted Chief Jefferson. "But new men will come up to take their places and the police department will keep functioning as it has in the past."

The retirement law was devised originally to encourage older law enforcement employes to consider retirement, partly as a way of ensuring that younger men would continue to have advancement opportunities, one police official said.