When President Reagan issued an executive order last month removing some law enforcement employes at the U.S. Marshals Service from union coverage, the American Federation of Government Employees was outraged.

Wallace Roney, president of the AFGE local that represents the marshals, called it "nothing but a game to bust the union."

The union warned that its 3,000 members at the Marshals Service would lose their health, dental and life insurance. The AFGE warned that its former members would have no grievance procedures for dealing with management. And the AFGE filed a motion Friday in U.S. District Court here seeking a temporary restraining order to block the move.

But administration officials say that the union's claims are seriously exaggerated. Marshals Director Stanley E. Morris said, first, that there could not be 3,000 union members at the service because "I have under 2,700 employes."

Informed of Morris' statement, union spokesman Loretta Ucelli said that the initial figure was wrong and that before the executive order took effect yesterday, the AFGE represented 1,872 employes at the Marshals Service.

Morris also disputed Roney's claims that all the marshals will be exempted from union coverage, saying that about half of those previously covered will remain eligible for union representation.

Morris added that most of the employes represented by the union are not dues-paying members. In fact, he said, only 278 of his employes were union members before Reagan's executive order. Only 100 of these will be eligible for union representation under the order, he said.

Told about Morris' figures, Roney said he is positive he has about 700 dues-paying members, but he is not sure how many will be affected by the executive order.

Morris said affected employes have the option of continuing to pay union dues to obtain the union medical and life insurance plans instead of switching to the Marshals Service health plans, although they cannot be represented by AFGE in union grievance procedures.

The major difference, he said, will be that the employes who want to obtain union benefits will have to pay their union dues by check rather than having them deducted automatically from their paychecks.

Nonunion employes, he said, can air their differences with management through the Marshals grievance procedures.

Morris said that the executive order, which was recommended by Morris and Attorney General Edwin Meese III, was an attempt to provide more flexibility with the criminal investigative employes.

Morris called the action an "inevitable step toward further professionalization of the agency," bringing it more into line with the FBI and the Drug Enforcement Administration, whose agents are not represented by unions.

But Loretta Ucelli, spokesman for AFGE, called the action a "direct attempt to deny representation to a group of employes who have done very well representing themselves through AFGE."

Roney said he believes the administration decided to exempt the employes because "the union has been very successful at fighting Meese and Morris." Roney added that AFGE just filed a nationwide unfair labor practices complaint against the Marshals because of a new cost-cutting move to introduce staggered working hours. Roney said the new system is inconvenient, especially for workers who have to deal with babysitters.