Arthur Allen Fletcher, a blunt-spoken former high-ranking black official in the Nixon administration, made a surprise announcement yesterday that he is running as a Republican candidate for mayor of Washington.

At a morning press conference at the new Harambee House hotel near Howard University, Fletcher said he knew his name was not well known among the overwhelmingly Democratic voters of the District, but added he hoped this handicap would be overcome with a heavy media campaign he plans.

Fletcher also said he would be seeking to galvanize Democratic voters who have "expressed anxiety over the current cast of (Democratic primary) candidates," a reference to the three front-runners in the race of the Democratic mayoral nomination - City Council Chairman Sterling Tucker and Councilman Marion S. Barry Jr. and Mayor Walter E. Washington who has arion S. Barry Jr. and Mayor Walter E. Washington who hacy soon.

Fletcher, who expects to run opposed for the Re he Sept. 12 Democratic primary in the November general election.

As assistant secretary of Labor, Fletcher created and, for a short while, aggressively implemented the Nixon administration's controversial "Philadelphia Plan" to increase the numbers of minorities employed in federal construction projects in cities across the country. The plan, first tried in Philadelphia, encountered strong opposition from organized labor that was led by AFL-CIO president George Meany.

After the furor over the Philadelphia plan, Fletcher was moved in 1971 from the Labor Department to the position of alternate U.S. representative to the United Nations. After three months there he left to become executive director of the United Negro College Fund. In 1976 Gerald Ford made him a White House urban affairs adviser. In recent months he has operated a private consulting firm here.

Fletcher said in an interview that initial seed money for his mayoral campaign will come form the Republican National Committee. Former California governor Ronald Reagan, former Texas governor John Connally and other national Republican figures have promised to help him raise the funds, he said.

Republican National Committee Chairman William Brock, a former senator from Tennessee, also would assist him in his campaign efforts, Fletcher added.

Reached in Easton, Md. yesterday, Brock said he was delighted with Fletcher's candidacy. "This is one race we take very seriously," Brock said.

Since Brock's loss of his Senate seat in the 1976 elections, he has been telling fellow Republicans that it is foolish for them to continue "writing off" the support of black voters, the vast majority of whom have been supporters of the Democratic Party since Franklin D. Roosevelt's terms in the White House more than three decades ago.

Under Brock, the Republican National Committee has set aside $700,000 to attract black voters away from the Democratic Party and to support black Republicans running in elections at the state, county and municipal levels.

Brock said the national committee is limited to a $2,000 direct contribution to Fletcher's campaign but "will be helping him in any way we can. We're going to have a slate of black candidates nationwide," he added.

Fletcher said he has received only "lukewarm and, at times, resentful reactions" to his mayoral candidacy from the majority of the 82-member central governing body of the District of Columbia Republican State Committee. "I have the active support of perhaps, a third of them," he said.

Of the 21,641 Republicans registered in the District as of February, almost half live in the mostly white area of the city - Ward 3 - west of Rock Creek Park. Citywide, there are 173,435 registered Democrats.

The only elected Republican official in the city is the Rev. Jerry Moore, an at-large member of the City Council. He said yesterday he has not thought about whether he will support Fletcher's candidacy. "I just received an invitation to his press conference," Moore said, "but no one has approached me and asked for my support."

Moore, who is member of the D.C. Republican Committee, volunteered that the group has not met to endorse a candidate for the mayoral race. Fletcher, who moved into Southwest Washington in 1975 from Columbia, Md., said he has not been active in local Republican politics.

The local Republican committee has the image of a defeated party organization dominated by staid, elderly conservative whites, political observers said. "The average age of its membership is around 50 years old," said one person intimately involved in local Republican politics.

Paul Hays, chairman of the District Republican Committee, said he is "actively supporting Fletcher's candidacy. I think he can win," Hays said.

During a two-hour interview in his town house Friday, Fletcher, dressed in a powder blue leisure suit, said he "was not worried about the lopsided registration of Democrats in this town."

He noted that in the Democratic primary race in 1974 between challenger Clifford Alexander and incumbent Mayor Washington, fewer than half of the 206,000 registered Democrats in the city voted.

Many of those Democrats who did not vote, Fletcher said, feel allenated and excluded from the District government. "I can generate a lot of feeling of participation and I'm going for a high turnout in the general election," he said.

Fletcher said he began to plan to run for mayor while serving as an urban affairs adviser to Ford. He has traveled to many cities, he said, noting urban problems and thinking about their solutions.

But when questioned about specific solutions he has for problems in Washington, Fletcher said, "I'll have to live with vague answers until I develop my positions."

He said eventually he hopes to be able to get local money for what he expects to be an expensive campaign. "I couldn't get the money here (now) because folks here don't want me to run here right now," he said.

Fletcher said he plans to soon start campaigning actively for the Democratic primary and will begin by discussing Mayor Washington's administration.

"It has been less than it could have been. That's going to be the real issue for the voters to decide - whether they want more of the same," he said.

After Ford's defeat, Fltecher established Arthur Fltecher Employment Standards Consultants Inc., a self-described "moon and pop" company that specializes in advising industry on implementing affirmative action programs for minorities.

Fletcher, 53, lives with his second wife, Bernyce, and two of their six children.

He grew up in Arizons, California and Kansas where he graduated with a bachelor's degree in political science from Washburn College in Topeka. For a short period he played professional football with the Los Angeles Rams, Baltimore Colts and the Hamilton (Ontario) Tiger-Cats in Canada. He came to Washington to join the Nixon administration in 1969 from the state of Washington where he had been active in Republican politics. While there, he had started a self-help business project in East Pasco, Wash. That venture was credited with influencing Nixon's ideas about "black capitalism."

Asked if he felt he would be charged by his political opponents as a recently arrived "carpetbagger," Fltecher said he has already been called that.

"Fifty or 60 percent of the population of the District is made up on people who weren't born here," he said. "And I'm going to organize an army of carpetbaggers and we're going to carpetbag our way right into the mayor's office. We're going to loosen this town up."