Incumbent Mayor Marion Barry led a Democratic rout in the District of Columbia's elections yesterday, winning a second term with about80 percent of the vote while Democrats triumphed comfortably in races for City Council and delegate to Congress.

Complete but unofficial returns showed a slim margin of approval for the proposed statehood constitution for the city, 59,300 votes to 52,177. An initiative calling for a nuclear weapons freeze was approved by more than two-thirds of those who voted, 77,521 to 33,369.

Barry, benefiting from the overwhelming majority that the Democrats have among voters in the District and a weak GOP challenge, enjoyed a 5-to-1 lead over his nearest rival, Republican E. Brooke Lee Jr.

In other races, D.C. Del. Walter E. Fauntroy, a Democrat, easily won election to a seventh term in Congress, receiving 76 percent of the vote.

City Council member David A. Clarke (D-Ward 1), 38, a former civil rights lawyer, was elected council chairman, after emerging from a tough, three-way Democratic Party primary contest with incumbent Arrington Dixon and former chairman Sterling Tucker.

Democrat Frank Smith, 38, a member of the D.C. school board, won election to the seat that Clarke will vacate in January, getting almost 80 percent of the vote.

Democratic incumbents easily won reelection in the other council races, including Polly Shackleton in Ward 3, William R. Spaulding in Ward 5, and Nadine P. Winter in Ward 6.

Incumbent at-large council members Betty Ann Kane (D) and Hilda Mason (Statehood Party) ran unopposed.

The Democratic sweep means that Democrats will retain their current majority in the City Council, where they hold 11 of 13 seats. One Republican and one Statehood Party representative hold the remaining seats. The winning candidates in yesterday's election will take office Jan. 3.

Winning D.C. Democrats joined more than a thousand people in a national Democratic Party celebration last night led by Democratic National Committee Chairman Charles Manatt at the Capital Hilton hotel. Barry spoke briefly and said that last night's election results were a preview of a tide that would carry a Democrat into the White House in 1984.

Later, Barry and Democratic Party State Committee Chairman Theodis R. (Ted) Gay gathered with other Democratic winners in District races, including Fauntroy, Clarke, Kane, Winter, Spaulding and Smith.

"The voters of the District of Columbia, like the voters of America, have issued today, effective November 1984, a RIF notice for Ronald Reagan," declared Fauntroy.

While Barry and local Democrats were on stage, losing Republican candidate Lee mounted the stage to shake hands with Barry and kiss Barry's wife, Effi.

In speaking to local Democrats, Barry said that soon he will reveal the priorities for his next adminstration and said that attracting jobs to the city will be a priority.

"Jobs must be the centerpiece of my next administration," he said. "It's not going to be easy. The District of Columbia suffers the same recessionary problems as the rest of the nation."

"The important thing is the big picture," Lee said last night before going to the Democratic celebration to congratulate Barry. "If Marion wins clearly as mayor, then E. Brooke Lee, a big businessman who knows how to bring jobs to Washington, will offer his services to bring jobs to the city.

"I don't see this as a gladiator fight to the death. It's a matter of what's best for the city, and if Marion's got the votes from his people, then we all ought to get together and do what's best to make this a happy, properous city."

Robert Carter, chairman of the D.C. Republican Party, said that the Democratic sweep should be viewed in context as local Republicans seek to build their party in the face of a 9-to-1 Democratic advantage in registered voters. He said that the party put most of its efforts this year into an attempt to defeat the proposed state constitution, including mailing out 50,000 flyers.

"We want to get behind issues that appeal to Republicans, concerned Democrats and independents," Carter said. "We did that purposely to see if people would go with their conscience, knowing that it was a bad document despite support from the mayor and the delegate."

Encouraged by a gloriously sunny Indian Summer day, voters had turned out in large numbers at precincts throughout the city. But with many voters assuming that most races virtually were decided, the scene at polling places yesterday was relaxed or subdued.

Barry's campaign strategy for the general election apparently had been to avoid campaigning. He did not attend three forums that had been scheduled with his opponents, and last week he declined an invitation to appear on television with the other candidates. Without Barry, the televison interview was canceled.

"I'm not crazy. I'm not going to put in 14 hours a day campaigning and not run the government," Barry said yesterday, outside a polling place that he visited at the Guy Mason Recreation Center, Wisconsin Avenue and Calvert Street NW. He said the voters had had ample opportunity to see him with the other candidates during the forums that preceded the Sept. 14 primary.

During the campaign that led to the primary, Barry had stressed his four-year record as mayor, telling voters that he had done much to encourage development in the District and to create jobs and affordable housing. He took skillful advantage of his incumbency while avoiding any major mistakes that could have cost him support.

He explained mistakes early in his administration, such as confusion in his handling of the city's budget crisis, by saying that since then he has learned and matured in the job.

Barry used the time between the primary and the general election in an effort to gain greater control of the D.C. Democratic Party.

Barry's aides chided committee chairman Gay for not having allowed the Democratic Party to endorse Barry during his primary race against former Carter adminstration cabinet secretary Patricia Roberts Harris and City Council members John Ray and Charlene Drew Jarvis.

With a strong campaign organization and a $1.2 million campaign chest, Barry won the primary with about 59 percent of the vote, easily besting Harris, who finished second with about 35 percent.

Republican candidate Lee had spent most of his campaign trying to engage Barry in debate. At one point, Lee took two mules, including one named Marion, to Western Plaza across from the District Building and held a mock debate with the animals.

Lee, brother of former Maryland acting governor Blair Lee III, campaigned on a platform of "More jobs, less crime," and said that he would use his connections with the executives of major corporations to persuade them to build plants in the District and hire local workers.

Sobin, the independent candidate, ran on a platform of civil liberties. He claimed that D.C. police and zoning officials had harassed and arrested him when he ran a club where sexual encounters were permitted.

White, the Socialist Workers Party candidate, ran a campaign denouncing the two major parties as "capitalistic tools," and said there was a need for a party that represents the working people.

Fauntroy, in his drive to retain the seat he has held for more than 11 years, also did little campaigning in the race with his little-known Republican challenger John West.

While most of the City Council races had been expected to be sure wins for Democratic candidates, Republican W. Ronald Evans had mounted a strong campaign against Ward 5 Democratic incumbent Spaulding for the right to represent the neighborhoods of Northeast Washington.