Mayor Marion Barry, rejecting advice that he abandon politics to concentrate on his addiction recovery, announced yesterday that he will be an independent candidate this fall for an at-large seat on the D.C. Council.

Barry's announcement, made one day after he quit the Democratic Party, was viewed by many as an act of political self-preservation that could jolt several city elections this fall.

Potential casualties of the mayor's decision are D.C. Council member Hilda H.M. Mason, a Statehood Party member and longtime Barry ally who is struggling to retain her at-large seat, and other independent candidates vying for the two at-large seats at stake.

Mason, 74, one of the longest-serving political activists in the city, said yesterday that she was "disappointed that {Barry is} going at this seat when there are other seats he could be aiming for."

Barry said in a statement read by a top aide that, if elected, "I can achieve my goal of continuing to make a contribution to the political life of this city without keeping the debate alive about my leadership of the city."

Barry, who faces sentencing for his misdemeanor conviction on one count of cocaine possession, ended speculation about his political plans one day after he registered as an independent to preserve his option of running in November.

The mayor missed the deadline for running as a Democrat while standing trial on drug and perjury charges. A federal jury found him guilty last week of one count of cocaine possession and not guilty of a second count. It was unable to reach a verdict on 12 other counts.

Responding to suggestions from some of his friends and advisers that he give up politics and focus on his recovery, Barry said, "I would like to remind those people that most men and women in other walks of life, including the media, are not expected to quit their professions after treatment.

"Instead, many people we know have continued in their jobs, often with clearer vision and making even greater contributions," said Barry, who served on the council for four years before he was elected mayor in 1978.

The mayor's statement was read by Anita Bonds, his top political adviser, before she picked up petitions from the D.C. Board of Elections and Ethics for Barry to appear on the general election ballot. Barry has until Aug. 29 to collect 3,000 signatures of registered voters to be certified as a candidate.

Barry said polls have shown that, despite his legal problems, he could win reelection as mayor by a plurality. He said he rejected that idea because "it would further polarize attitudes and further alienate certain segments of the population from each other."

Mason, who has been allied with Barry since she joined the council in 1977, said she was not "sad or bitter" over Barry's action, merely disappointed.

Mason, chairwoman of the council's Education Committee, added, "I think I have done a lot for the District of Columbia. I'm going to win this struggle."

Three Democrats are vying for an at-large post: school board member Linda Cropp, former congressional aide Johnny Barnes and housing activist Terry Lynch.

The winner of the Sept. 11 Democratic primary is likely to be favored to win one of the two at-large seats in the general election. No Democratic nominee has lost a general election in the city since Home Rule in 1974.

That would leave Mason, Barry, Republican candidate W. Cardell Shelton and four independents -- Jim Harvey, Ray Browne, Clarene Martin and R. Rochelle Burns -- competing for the other at-large seat.

In such a divided field, Barry would stand an excellent chance of winning a plurality, activists say, despite polling data suggesting widespread opposition to his further participation in politics.

Harvey, a deputy director of the Whitman-Walker Clinic, said he was "in no position to argue" with Barry's action, and said he "welcomes the mayor's entry into the race."

Barnes said it would have been better "for his family" had Barry not entered the race, but added, "That won't change my strategy at all. I intend to continue to run as a candidate of fresh approaches."

In his statement, Barry disputed published suggestions that he would be a "de facto mayor," with independent ties to the city bureaucracy. "I believe in a strong executive form of government, and am quite willing to let a new mayor be mayor," he said.

The mayor needs four more years of D.C. government service to qualify for a 20-year pension. He dismissed those who have suggested that money was a factor in his decision as "insane."