A union claiming to represent 2,000 civilian Army employes at Fort Belvoir has sued the government, alleging that the Army improperly has hired contract employes to work at the base, an action that the union says will result in the layoffs, transfers, demotions or the forced retirements of hundreds of workers.

Local 1052 of the American Federation of Government Employees, AFL-CIO, charged in a lawsuit filed this week in U.S. District Court in Alexandria that the Army's recent $5.5 million, one-year contract with Dynalectron Corp. of McLean will eliminate 531 jobs at the Fairfax County base by early next year.

In the 11-page complaint, union officials also accuse the Army of consistently overestimating the cost of having the affected work performed by federal employes and underestimating what it would cost if done by a private contractor. That is a violation of federal contracting laws, the union says.

A Fort Belvoir spokesman yesterday disputed the union's claims, insisting that "no permanent employes should lose their jobs" as a result of the contract. Some temporary ones will, said spokesman Jerry Childress.

He also said 273 employes have been notified that they will be transferred to other positions at Fort Belvoir or elsewhere. Childress could not say how many temporary employes would be afffected, and it also was unclear whether the employes would be transferred to similar positions or ones that would result in pay cuts.

Most of the jobs involved are those covered by the base's directorate of engineering and housing, which overseas the maintenance of all base buildings, according to the union's attorney Steven Cohen.

Janitors, technicians and specialized mechanics are among those affected, Cohen said.

He called the contract a "Reagan administration special" that puts federal employes out of work while allowing the adminstration to take credit for new jobs in the private sector.

"But it's costing the government more," Cohen said. "It's just coming out of a different pocket."

According to union officials the Army figures cited by Childress do not take into account the numbers of employes who have retired from the post or left their jobs to seek employment elsewhere after the contract was announced last spring.

"As soon as they announced Dynalectron won the contract, people started bailing out," Cohen said.

The contract includes provisions for an extension of up to four years.

Union officials have also criticized the Army for releasing few details about the impact of the contract award, and said that they have filed two unfair labor complaints against the Army with the Federal Labor Relations Board this month.

Union officials have asked the Alexandria court to ban the company from starting work at the base and to rescind reduction-in-force and job-change notices pending arbritration hearings that could be held next spring. A federal court hearing on the union's request has been scheduled for Dec. 21.

Fort Belvoir officials declined to comment yesterday on the charges of overestimating and underestimating labor costs.