In the summer of 1966, when the Southern Christian Leadership Conference organized a March Against Fear across Mississippi, Robert L. Green, then an assistant professor on leave from Michigan State University, was often at the head of the line.

In Grenada, according to news reports, Green climbed on a memorial to Jefferson Davis and planted an American flag atop the Conferedate president's head.

"We're tired of rebel flags," Green shouted. "This is the flag we want to see." A few minutes later, Green told the blacks in the crowd to begin testing the federal public accommodations law by using the "whites-only" toilets in the courthouse.

"Bob Green is my idea of the scholar activist," Del. Walter Fauntroy (D-D.C.) said yesterday after Green was named president of the University of the District of Columbia.

"I think of Green as a scholar because he's thorough in his examination of what needs to be done," continued Fauntroy, who also participated in that Mississippi March led by the Rev. Martin Luther King Jr. "He's an activist in that it's his orientation to do it."

During the past 10 years, as a dean at Michigan State, Green has published about 100 scholarly articles plus a textbook called, "The Urban Challenge--Poverty and Race." Many of the articles deal with the major controversies of race and education, with Green often questioning the fairness of standardized tests and championing the virtues of school desegregation.

Green has stepped out of the classroom to serve as a consultant and expert witness for the NAACP in more than a dozen major school busing cases. Recently, he was president of a firm seeking a cable television franchise in Detroit.

He was selected as president of UDC Tuesday night by a unanimous vote of 13 members of the university board of trustees. His predecessor, Benjamin H. Alexander, an outspoken advocate of tough academic standards and administrative cutbacks, resigned under pressure a month ago after less than a year in office.

Green will become president Sept. 1 on a five-year contract with a starting salary of $74,500 a year. He will start work as a consultant on Aug. 1.

At a press conference yesterday he declined to discuss Alexander's criticism of the university, but his emphasis was somewhat different from that of his predecessor.

As the audience of about 75 faculty members, administrators and trustees applauded, Green declared: "My goal is not to look to strategies to make life difficult for students but to look to strategies to help them succeed."

Later, he said, "You would not find a university president alive today who does not advocate high standards. I advocate them for Robert L. Green and everyone else. The problem is to help people reach them . . . . I don't want to get caught in the position of saying things that are not supportive of the university."

In many of his speeches Alexander inveighed against "coddling . . . so-called disadvantaged students."

Green said he wants "the highest possible level of education at the university," but he added, "The majority of racial minority students today have not always had the opportunity to succeed in education. We know the history of segregation. We know the history of dual school systems."

UDC has about 14,000 students. More than 90 percent of them are black and many come from low-income families. Almost three-quarters are placed in remedial, high-school level courses.

"Almost every university today has a major tutorial program and a type of open admissions policy," Green said. "I think they can work . . . . "

At Michigan State Green served from 1973 to 1981 as dean of the college of urban development, which had at a maximum about 120 students and eight full-time faculty members. But when the state encountered serious budget problems, the university trustees abolished the college of urban development despite strong statements from Green and from black leaders in Michigan.

He was then made dean of urban affairs programs, in charge of research and service programs but without direct authority over courses and students. Although he retained his salary, currently $57,550, friends said his outside interests expanded.

He became president of City Communications Inc., a firm set up to bid for the Detroit cable TV franchise. But last week Mayor Coleman Young endorsed a competing firm, which is expected to get City Council approval. On Tuesday Green said he had resigned as company president but will continue as chairman of its board.

Green, 49, was born and raised in Detroit, the son of a Pentecostal minister with nine children. After graduating from public high school, he served in the army and took bachelor's and master's degrees from San Francisco State College before moving to Michigan State for a doctorate in educational psychology.

Yesterday Green said he plunged into the civil rights movement in the mid-1960s after hearing Martin Luther King Jr. speak. He has never left it and counts as his close friends Coretta Scott King, the widow of the slain civil rights leader, and Andrew Young, the mayor of Atlanta.

"He's one of the few activist academics I know," Young said of Green yesterday. "He's got the credentials and he's got the commitment." CAPTION: Picture, DR. ROBERT L. GREEN . . . starts at $74,500 a year.