Rep. Parren J. Mitchell, the Baltimore Democrat and civil rights advocate who was elected in 1970 as Maryland's first black member of Congress, will not seek reelection next year, political sources said last night.

Mitchell, 63, a member of a family prominent in the civil rights movement and in Maryland politics, declined when reached by phone to confirm that he will not run for an ninth term, and he said he plans to mail a statement today.

He had indicated in 1984 that he was leaning toward retirement, but he said he ran because of strong backing from friends, family and community leaders.

Mitchell's decision comes at a time of quickening activity on the Maryland political scene. Rep. Marjorie Holt (R-Md.) announced recently that she would not seek reelection to her seat representing the 4th District, which includes Anne Arundel and parts of Prince George's and Howard counties. And last week Sen. Charles McC. Mathias (R-Md.) said he will not seek another term.

At least five Baltimoreans were being mentioned last night as possible successors to Mitchell.

Chairman of the House Small Business Committee and a former head of the Black Congressional Caucus, Mitchell has been a vigorous spokesman in both Republican and Democratic administrations for the rights of blacks and minorities.

While serving in 1978 as head of the Black Caucus, Mitchell recognized that political tides were shifting away from his positions. "Only a fool would fail to notice there has been a shift to the right," he said in assessing that year's election returns.

But he declined to shift his own position.

"I am not ashamed," he said in 1980. "I am a liberal and I always will be."

On Capitol Hill, Mitchell worked for years to assure minority participation in contracts let under federal public works programs.

He was an original sponsor of legislation approved in 1977 that guaranteed minority contractors a share in public works spending that amounted then to about $4 billion a year.

He also was the sponsor an an amendment to the 1982 Surface Transportation Assistance Act stipulating that at least 10 percent of the funds provided under the law should go to small businesses owned and controlled by socially and economically disadvantaged people.

As head of the Black Caucus during the Carter administration, he was a leader in calling on the government to oppose the South African apartheid regime.

During the 1980 presidential campaign, he called Ronald Reagan a "clear and present danger to black Americans."

"I am angry," he declared in the fall of 1982 after figures showed unemployment at its highest level in more than 40 years, and he called Reagan "this dreadful man."

In addition to serving as chairman of the Small Business Committee, he is a senior member of the Banking, Finance and Urban Affairs Committee and has been described as the legislative guardian of minority business.

A Baltimore native who received a bachelor's degree from Morgan State in Baltimore and a master's degree from the University of Maryland, Mitchell was an assistant professor of sociology at Morgan when he ran for Congress in 1970.

He defeated a veteran incumbent by 38 votes in the 7th District primary. In recent years his seat has been one of the most secure in the House.

In 1978, police arrested three men allegedly involved in a scheme to bomb his home. "There's always some nut out there," he said. "You just have to be prepared for these things."

Mitchell is the brother of the late Clarence Mitchell Jr., who was chairman of the Leadership Conference on Civil Rights and a guiding force in passage of much of the major civil rights legislation of the 1960s.

The race to succeed Mitchell is expected to include many prominent Baltimore names, including two nephews, state Sen. Clarence Mitchell III and City Council member Michael B. Mitchell, Council member Kweisi Mfume, attorney A. Dwight Pettit and state Del. Wendell Phillips, who is chairman of Baltimore's House delegation in Annapolis.