Mayor Marion Barry will formally announce his campaign for a third term next Saturday at the Washington Convention Center, according to a campaign official.

Earlier this week the mayor's campaign committee opened a headquarters in a suite in a downtown office building at 733 15th St. NW, according to Anita Bonds, the mayor's campaign manager.

Bonds said she expects the mayor's formal announcement to provide a lift to the reelection effort.

Unlike four years ago, when six major candidates had challenged the mayor by April, the absence of a serious major opponent this year has slowed fund-raising efforts, according to Barry and his aides.

"We're raising money slowly," Bonds said, "but we need an announcement to make a difference. It will help generate interest and enthusiasm. People need to know there is a campaign."

Bonds declined to say how much the mayor has raised beyond the nearly $150,000 his committee disclosed in a report filed with the D.C. Office of Campaign Finance in early March. The next report is required to be filed in June.

Aides to Barry and two other announced mayoral candidates -- Mattie Taylor, a former school board member and D.C. Department of Employment Services official, and Dennis Sobin, who publishes a sex-oriented newspaper -- picked up petitions from the D.C. Board of Elections and Ethics yesterday to qualify to appear on the Sept. 9 primary ballot.

The petition process requires Democrats seeking the mayoral nomination to obtain 2,000 signatures of registered Democrats by July 2. Both Sobin and Taylor are seeking the Democratic nomination.

Two others have formally announced for mayor -- Brian Moore, a health care consultant, and Calvin Gurley, an unemployed accountant. They are running as independents and must wait until July 14 to pick up petitions to qualify to appear on the November general election ballot.

D.C. City Council Chairman David A. Clarke, who had considered running for mayor, has decided to seek reelection to the chairman's post. Two other council members who have been weighing a bid for mayor, John A. Wilson (D-Ward 2) and Charlene Drew Jarvis (D-Ward 4), have not yet formally announced their intentions and did not pick up petitions yesterday.

Among others who picked up petitions were former council member Douglas E. Moore, who is expected to announce his candidacy for council chairman today; council member Frank Smith (D-Ward 1), who is seeking reelection; Elona Evans-McNeill, a Democratic Party activist who has said she is considering opposing Smith, and council member Hilda Mason (Statehood At-Large).

In Ward 5, where council member William R. Spaulding is expected to seek reelection, Harry L. Thomas, a Democratic Party activist, picked up petitions to oppose Spaulding, as did Mark Chao Sung and Ozey Thorpe.