In a concrete-block building, behind a barbed-wire fence that used to guard the Defense Intelligence College, officials from the Pentagon and General Services Administration are scurrying to get ready for the 50th inauguration of an American president.

Since spring, uniformed members of the Armed Forces Inaugural Committee (AFIC) have been working side-by-side with civilians from the GSA to position support staff and prepare office space for the more than 500 employes and volunteers expected to work for the Presidential Inaugural Committee.

The privately funded Presidential Inaugural Committee won't spring to life until Nov. 7, the morning after the election. It is responsible for the events other than the inaugural ceremony itself -- the receptions, parade and the balls. The ceremony is handled by the congressional Joint Committee on the Inaugural.

The AFIC, meanwhile, is responsible for running the inaugural command post, advising the Presidential Inaugural Committee and shepherding the more than 8,000 soldiers who will participate in the inaugural activities. Its job includes organizing the 1,200 soldiers who will form a cordon along the inaugural parade route, coordinating intelligence gathering, providing medical facilities and designating personal ushers for dignitaries.

"The military role is traditional," said Brig. Gen. William A. Roosma, deputy chairman of the committee. "We've grown from a very small Washington celebration to a worldwide one. For a democracy, this truly portrays the peaceful orderly transition of power." The military officers view their participation as appropriate since the inauguration, in effect, is also the installation of their commander in chief.

The GSA's job is to set up the offices and provide the telephones and supplies to keep them running for three months.

"The dimensions of the actual inaugural events remain up to the winner," said Ted Leininger, deputy regional public buildings commissioner, who is working on his fourth inaugural. "What we have here is the physical framework for an inaugural committee to operate."

The inaugural committee's headquarters on the Anacostia River were supposed to have been razed last spring, until the GSA discovered it had no place to put the offices. Now the old intelligence college has been stocked with refurbished wooden desks culled from GSA's furniture stockpile.

Each desk comes with a chair, telephone, pen and pencil, note pad, calendar and wastebasket. Other furnishings, including bookcases and additional chairs, will be provided as needed. But equipment such as typewriters and photocopying machines will have to be purchased with private funds.

Roosma said that the AFIC has been working with the GSA to prepare a list of activities that the Presidential Inaugural Committee can choose from, including such options as fireworks displays and moonlight cruises on the Potomac River.

"Too often," he said, "we've repeated the same mistake . . . that we had made before." To eliminate "the problem of reinventing the wheel" in the future, Roosma said, AFIC is preparing a multivolume planning guide for inaugural activities, based on the 1985 event.

Representatives of both President Reagan and Democratic candidate Walter F. Mondale have visited the compound and viewed a slide show on the services that will be available. Leininger said White House officials have indicated that if Reagan is reelected, his supporters would not try to outdo their efforts in 1981, when the inaugural events cost $16.3 million, a record.

"They said they'd scale it down," Leininger said. "But we don't know yet what that means." As for the Democrats, he said they "have never gone overboard on inaugurals."

After the 1981 event, Sen. William Proxmire (D-Wis.) asked the General Accounting Office to determine whether the Defense Department had wasted money on its preparations.The auditors questioned whether DOD had the statutory authority to provide military chauffeurs for some inaugural guests and to billet nonmilitary participants in local military installations, among other things. The GAO recommended legislation to clarify the lines of authority, but so far no legislation has been passed.

Roosma said this time the activities will "be as extensive as in the past, but we're being much more cost-conscious."

Officially, the federal share of the activities is expected to cost just over $2 million -- DOD says it will spend about $550,000, while the GSA has a special $1.5 million appropriation. But GSA bills everything to the special account, while DOD covers salaries from other accounts.

"We're far better organized than we were in 1980-81," said Col. Michael A. Dickerson, director of public affairs for AFIC, who is now working on his sixth inaugural. He said the difficulties usually crop up on short notice, such as when President Carter decided to walk from the U.S. Capitol to the White House in 1977, creating new security problems, or when demonstrators threatened to disrupt the parade planned for President Nixon in 1969.

The parade usually presents the most problems to the AFIC. It may have to bus dozens of marching units from the the AFIC headquarters to Capitol Hill, move floats from the Washington Navy Yard and horses with equestrian groups from stables at Rosecroft Race Track.

But the biggest threat, Dickerson said, is always the weather.

Organizers still look back with pride at John F. Kennedy's inauguration in 1961, when they cleared eight inches of snow from the parade route in time for the festivities. Inaugural planners figure they would need 1,000 dump trucks if that much snow fell next January -- and they're still trying to find them.