Sterling Tucker, a reserved but canny politician who once directed the Washington Urban League and is now chairman of the District of Columbia City Council, said yesterday that he plans to run for mayor in the Sept. 12 Democratic primary election.

The 53-year-old Tucker, who first built a reputation as a civil rights leader in the city and later emerged as an effective and moderating force in the city's infant home rule government, said a formal announcement of his long-expected candidacy will be made in early March.

Tucker took a first step toward that candidacy yesterday by registering as a contender for mayor with the D.C. Board of Elections and Ethics and also informing the board that he has authorized a campaign committee to raise and spend money on behalf of his candidacy.

"There is no doubt in my mind," Tucker said at an afternoon press conference in his Council office, "that I am going to run mayor."

It was the Council chairman's first public acknowledgement of a candidacy that has been evident in city political circles for months. Tucker was the second of three well-known city politicians who are expected to run for mayor to make such an informal announcement.

On Oct. 26, City Councilman Marion Barry said that he too planned to seek the Democratic mayoral nomination this fall. Barry is expected to formally announce his candidacy on Saturday.

Mayor Walter E. Washington has not decided if he will seek election to a second term.

Tucker's registration as a candidate allows him to begin collecting and spending money openly on the mayor's race, something neither Barry nor Washington can do without similarly declaring their intentions.

Barry has been cautions about soliciting and collecting campaign funds because of a city law that would require him to resign his Council seat as soon as he becomes a candidate for another office, Barry's term expires in 1980.

Washington, on the other hand, would not have to resign if he became a candidate, since his mayoral term, like Tucker's Council chairmanship, expires in 1978 - the same time that the term for the office he would be seeking is sheduled to begin.

All three have been running what many view an unofficial campaigns for several months. Numerous city businessmen and professionals have said privately that they have been approached by the candidates for pledges of support. Many of those approached consider those overtures as a request for campaign contributions.

Tucker enters the mayoral race with the formal endorsement of Del. Walter E. Fauntroy, (D-D.C.), one of the city's most popular Democratic politicians who is believed to have one of the most effective political organizations in the city.

Part of that organizational base is shared however, with Barry, who along with Tucker and Fauntroy led a citywide slate that seized control of the Democratic State Committee in 1976.

Fauntroy and other Tucker supporters tried for months to persuade Barry not to challenge Tucker for mayor, saying that Tucker was more experienced and had already served as Council chairman. They also said a Barry mayoral candidacy could lead to a split in the ranks of the state committee, which could result in a victory for Washington.

Many of the details of Tuckers impending campaign were still unclear yesterday, including who - if anyone - he will endorse as his candidate (or running mate) in the race to succeed him as Council chairman. He did say, however, that Councilman Arrington Dixon (D-four) and the Rev. David Eaton, senior minister at All Souls Unitarian Church, are among those under consideration.

Tucker said he wanted to talk more with people from various sectors of the community before announcing his platform. Generally, he said yesterday, his candidacy will be aimed at more efficiency in government, better delivery of city services, comprehensive city planning, redevelopment and providing housing and a clean environment in Washington.

Tucker said that his experience in city government and his work in the community made him a qualified candidate for mayor. "I believe I can bring all this together in a plan and a program which can help us achieve a higher vision and realize some of the goals that are possible," Tucker said.

Tucker came to Washington in 1956, from Canton, Ohio, where he was executive director of the Canton Urban League. Tucker was Washington Urban League director until 1974, when he resigned to run for City Council chairman.

Through his work with the Urban League. Tucker became one of the recognized spokesmen for the city's black community. He was a member of the appointed D.C. City Council and for a time served as its vice chairman. In 1974, when the city elected its first local government in more than 100 years, Tucker ran virtually unopposed for Council chairman and won overwhelmingly.

Tucker more than anyone else in given credit for the successes of the Council, which took office in 1975. With few of its members experienced in local government, many businessmen feared that a legislative body made up largely of former community activists would be punitive, irresponsible and ineffective.

But under Tucker's direction, the Council, after some stormy times during its first few months in office, is now viewed by many business and civic leaders as far more predictable and broad-minded than once expected. At the same time, may community activists accuse the Council of having sold out to business interests.

During Tucker's career in local politics, there have been three accusations of financial impropriety on his part, only one of which involved a conviction.

In 1958, a federal grand jury in Cleveland indicted Tucker on a charge of evading payment of $1,700 in taxes. Tucker pleaded no contest to the charge paid a $500 fine and received a one year suspended sentence. On Christmas Eve in 1968, he was pardoned by President Lyndon B. Johnson.