Supporters of Patricia Roberts Harris took the first official step toward launching her candidacy for mayor yesterday by registering a fund-raising committee for the Sept. 14 Democratic primary.

According to papers filed with the campaign finance office of the D.C. Board of Elections and Ethics, Sharon Pratt Dixon, Democratic national committeewoman, heads the Pat Harris for Mayor Committee. Its treasurer is Charles T. Duncan, a lawyer now in private practice who served as treasurer of former mayor Walter E. Washington's 1974 campaign and is a long-time friend of the former mayor.

Barbara Clark, vice chairman of the Democratic State Committee, is listed as the committee's custodian of books and accounts. They are the only three persons listed as members of the committee on the registration form.

Under the city's election law, Harris' supporters are now legally able to raise money for her campaign but within five days of the first contribution Harris must file as a candidate.

Marilyn Robinson, Harris' spokesman, said Harris will have a written announcement on her candidacy next week.

"Money has already been raised," said Dixon, "It's coming from a number of sources so it is hard to put a number on it right now, but it is more than necessary to get started and for her to feel comfortable with."

Duncan said he spoke with other candidates including Mayor Marion Barry before making the decision to work for Harris. "I've known Pat for a long time," Duncan said. "She's a person with proven ability and integrity, and based on her previous many experiences in and out of government she is immensely qualified to manage this city and manage it very well."

While Harris must formally file with the Board of Elections and Ethics within five calendar days if the committee accepts any money, she also has the option--if she decides the amount of money raised is insufficient--to write the committee in five days, by next Tuesday, and instruct them to collect no more money for her because she is not a candidate.

Dixon, who said she will not act as the manager of Harris' campaign, said Harris is aware that the group registered yesterday. She said Harris' supporters do not anticipate any problem with fund-raising that would derail the campaign.

Harris was unavailable for comment.

The former Carter cabinet secretary has been considered a likely entrant into the city's mayoral race since last year when a poll was done by a group of her supporters to evaluate her chances against Mayor Barry. While she has spoken to groups around the city in the last three months, Harris has been slow to say whether she will run.

Dixon characterized Harris as "cautious" in preparing every aspect of her campaign before making a formal announcement of her candidacy.

Those who have already filed as candidates for the Democratic mayoral primary include Barry, council members Betty Ann Kane, John Ray and John Wilson, physician Morris Harper and Richard Jackson. Two candidates, James Edward Champagne and E. Brooke Lee, have filed for the Republican primary.