Mayor Marion Barry, flush from a reelection victory in which he captured nearly 80 percent of the vote, is faced with the task of rebuilding his administration practically from the ground up.

To his advantage, Barry, confident and relaxed as he faced the press yesterday at the District Building, improved on his 1978 margin of victory by 11 percentage points. He has had nearly four years' experience in office. And he will be working with a new City Council chairman, David A. Clarke, who is a political ally, not someone expected to use the post to run against Barry.

But the rebuilding task confronting him is awesome. City Administrator Elijah B. Rogers, one of his most trusted aides, is expected to leave early next year. The directors of the Department of Finance and Revenue, a key post in a time of fiscal austerity, and the Department of Human Services, the city government's largest agency, have announced they intend to depart. And one-fourth of the heads of city agencies are only acting directors.

The mayor is considering far-reaching proposals to replace the city administrator with one or more deputy mayors and to cut down on the number of people who report directly to him, a source said.

Also, Barry announced yesterday that Ivanhoe Donaldson, his campaign manager and top political adviser, is heading a 28-member transition team reviewing every aspect of city government to recommend changes that would improve services and cut down on costs.

"We're looking at everything -- nothing is sacrosanct in terms of the structure," Barry said during his post-election press conference. "But what I want to do is be able to govern in a pro-active manner. . . taking steps to move us ahead of the curve as opposed to behind the curve. And whatever structure best suits my own personality, my own management style, my own philosophy, we'll do."

Later he added, "We may decide to restructure even if he Rogers stays. But the public isn't interested in whether we restructure . . . You can have the best structure on paper, but the citizens out there paying all these taxes want increased services."

Barry, who drew more than five times the votes of his nearest competitor Tuesday, Republican E. Brooke Lee Jr., acknowledged that his near-obsession with the District government's budget problems in 1979 and 1980 seriously impaired his ability to reduce crime and unemployment. He acknowledged that his administration did less than it could have in those areas.

"The budget was the tail that wagged the dog," Barry said. "I think we spent too much time on the budget . . . We finally got it under control and now we're in charge of it, we're on top of it."

But those financial and budgetary problems are likely to persist during his next term, as government costs continue to mount and tax revenues stagnate because of the decline in the economy and falling property values.

Barry would not completely rule out the possibility of an increase in business and property taxes during the next four years, although he asserted, "I'm not at all interested in advocating any major tax increases."

He contended that a revenue shortfall could be alleviated partly by more aggressive collection of taxes owed the city and by achieving greater cost savings in government.

"I'd be less than honest if I did not admit and say to the public that the District of Columbia is going to be the victim of the national economy as other cities are, and I'm going to have to alter some of my thinking about how we proceed in the light of reality," the mayor said.

"I don't believe that government should do everything for everybody," he added. "I believe that government should provide the basic essential services to people and then to look at those who are vulnerable."

At the same time he is trying to rebuild his government, Barry has to mend a politically sensitive and crucial relationship with Clarke, the new council chairman.

Barry, whose campaign provided assistance to Clarke in defeating incumbent Chairman Arrington Dixon, is counting on Clarke to help him ease his policies through the City Council. Barry was often angered by the Dixon-led council, which constantly battled him on budget issues and spawned four challengers to Barry in the Democratic mayoral primary.

Dixon himself frequently acted as though he were preparing to challenge Barry for mayor. Clarke is considered a safe chairman by Barry, said Barry aides who asked not to be named, because he is white and they doubt that any white candidate could win a mayor's race in a city that is 70 percent black.

But yesterday, Clarke said he believes that kind of talk from Barry's camp is intended to remind him that he is locked into Barry's orbit. Clarke added that he is not currently interested in running for mayor, but that if he had been interested earlier this year he could have formed a coalition of labor and tenants' groups that could have challenged Barry.

"Marion didn't get me elected," Clarke said yesterday. "I don't owe Marion anything from the campaign. We do serve a lot of the same people and made some of the same pledges. During the campaign I said I'd work with the mayor, without endorsing anyone for mayor. That's worked for me in the past, and I'll keep using it."

Barry's aides contend that Barry has enough support among council members to block Clarke on committee assignments and key legislative votes. Clarke declined to say yesterday whether Barry has talked to him about committee assignments.

"My decisions have been and will be what's right for most people," Clarke said. "When disagreements occur I will have to assess what compromises are most effective to the end I'm trying to achieve. I have made no commitment to do what Marion wants."

Most observers agree that Donaldson, the mayor's adviser and former acting director of the Department of Employment Services, is the moving force behind Barry's administrative reorganization and reshuffling of personnel.

"Ivanhoe is like a 500-pound gorilla," one high-ranking D.C. official said yesterday. "He's frankest with the mayor. He's the one who can tell the mayor when he is making a mistake."

As head of the transition team, Donaldson has appointed eight task forces to recommend changes in Barry's approach to jobs and economic development, criminal justice, education, human services, regulations, housing, public works and the inner working of the mayor's staff.

Thomas Downs, acting director of the Department of Transportation, is heading the task force on economic development--an area that Barry has given the highest priority. Downs has been mentioned by sources as a possible candidate for a top post in the new Barry administration.

Other members of the transition steering committee include some of Barry's top aides and closest supporters, including lawyer Robert B. Washington Jr.; advertising executive David Abramson; lawyer and fund-raiser Max Berry; mayoral special assistant Courtland Cox; mayoral executive secretary Dwight S. Cropp; C&P Telephone executive Delano Lewis; University of the District of Columbia trustee Lorraine Whitlock, and Greater Washington Board of Trade President Stephen Harlan.