Defiantly turning her back toward the judge through most of the proceeding, accused terrorist Marilyn Jean Buck was arraigned today on fugitive, racketeering and conspiracy charges stemming from the 1981 Brink's armored-car robbery in which three persons were slain.

Buck, 37, was arrested Saturday in Dobbs Ferry, N.Y., with fugitive Linda Sue Evans, 38, who was arraigned today on charges of harboring Buck.

Buck, a fugitive since 1977 when she failed to return to federal prison in West Virginia while serving a 10-year term for buying weapons and ammunition for the Black Liberation Army, has been described by law enforcement officials as the group's only white member.

She is suspected of driving a getaway car in the $1.6 million Brink's robbery in Nanuet, N.Y., on Oct. 20, 1981, when two Nyack, N.Y., policemen and a Brink's guard were shot to death.

Buck, believed to have shot herself in the leg accidentally in the getaway, also faces New York state charges of murder and robbery in the Brink's case and New Jersey charges in a 1978 armored-car robbery and a 1979 holdup.

FBI officials said she is "the key figure" and main communication link between the BLA and other radical groups, including the Weather Underground, Black Panthers and the Republic of New Afrika.

Buck is also under indictment for allegedly participating in the 1979 New Jersey prison escape of terrorist Joanne Chesimard, a BLA leader convicted of killing a New Jersey state trooper.

In U.S. District Court in Manhattan today, Judge Kevin T. Duffy entered a plea of not guilty for Buck after she refused to plead unless fellow radical Susan Rosenberg were present. Rosenberg, 29, is awaiting sentencing after being convicted in March for possession of explosives, weapons and fake identification cards.

"This is an attack on the New Afrika Liberation Movement," Buck said. "You can't talk about individuals. There will be no plea until my comrade is produced."

A score of supporters stood and applauded as Buck entered the crowded courtroom, limping slightly. She raised a fist in salute. Smiling defiantly, she swiveled her chair to face away from Duffy.

Assistant U.S. Attorney Patricia Williams urged Duffy to hold Buck without bail as a danger to the community and someone who might flee if released. Williams cited Buck's failure to return to the West Virginia prison after being given a furlough to consult with her attorney, Susan Tipograph, here. Tipograph represented Buck today.

"Mr. Duffy, I am an anti-imperialist freedom fighter," Buck told the court to loud cheers. "I am not a danger to the community of peace- and freedom-loving people."

Duffy remanded Buck to the Metropolitan Correctional Center, citing the warrant for her prison escape.

Grinning broadly, Buck raised her fist as she left the courtroom and shouted, "Free the Land," a slogan of the Republic of New Afrika, which seeks to establish an independent nation of blacks in the southern United States.

Buck was convicted in San Francisco in 1973 for buying weapons and ammunition for the BLA.

During the trial, authorities described her as the group's "quartermaster" in charge of procuring weapons, ammunition and false identification papers.

Authorities say that Buck, with Evans' help, evaded arrest since 1977 by moving within a network of safe houses in Baltimore, New Haven and the Bronx.

The 1981 Brink's robbery led to murder convictions and 75-year prison sentences for radicals Samuel Brown, David Gilbert, Judith Clark and Kuwasi Balagoon and a 20-year term for former Weather Underground leader Kathy Boudin. Convicted on related charges were Sekou Odinga, Silvia Baraldini, Cecil Ferguson and Edward Joseph.

Chesimard and Mutulu Shakur, alleged mastermind of the Brink's holdup, remain at large