In a surprise move yesterday, Cathlyn Platt Wilkerson, a former leader of the revolutionary Weather Underground, pleaded guilty of possession of dynamite, saying she wanted to avoid "a sensationalized trial" and was ready to begin "above-ground political work."

In exchange for the guilty plea, Manhattan District Attorney Robert M. Morgenthau dropped a charge of criminally negligent homicide. Wilkerson had originally pleaded innocent to both charges, which stemmed from the 1970 explosion of a bomb factory in her father's Greenwich Village townhouse that killed three Weathermen.

The dynamite possession charge could bring as many as seven years in prison when she is sentenced Oct. 24.

"I am anxious to begin a new life, having personally experienced difficulties over the past few years," Wilkerson said in a statement released by her attorneys at the Center for Constitutional Rights in New York.

Daughter of a wealthy advertising executive and a graduate of Swarthmore College, Wilkerson became one of the best-known radicals of the 1960s. After ten years in hiding following the explosion, she surrendered to police on July 8, but has refused to elaborate on her reasons or on her whereabouts for the past decade.

The Weather Underground is a group of several dozen revolutionaries who took their name from the words of Bob Dylan song, "You don't need a weather man to know which way the wind blows." They claimed responsibility for 25 bombings including the Pentagon in 1972 and the State Department in 1975. Many members, including Bernadine Dohrn and Kathy Boudin, daughter of civil rights attorney Leonard Boudin, are still underground.

Wilkerson said she decided to plea-bargain because "I do not want to discuss my own deep regret over the deaths of three beloved comrades in the context of a sensationalized trial." She added that her father, 65, who now lives in England, had been "threatened with a grand jury subpoena, I don't want to expose members of my family, my friends and my associates to such personal and political harassment."

In her statement Wilkerson said she decided to plead guilty despite her belief that conviction would be unlikely due to illegal wiretaps and burglaries that FBI officials have admitted undertaking in their effort to hunt down Weathermen. The Justice Department is prosecuting several FBI officials for allegedly engaging in such activities.

Also, she added, a conviction would be difficult to obtain because of "the climate of the time that motivated me and others to take the actions that we did -- for example, the active U.S. government role in murders in Vietnam and of leaders of the black movement here."

Wilkerson, now 35, first became politically active as a college freshman when she joined a 1962 picket line to protest segregation in Cambridge, Md. She worked in the civil rights movement and in the antiwar movement, where she was the Baltimore-Washington regional coordinator for Students for a Democratic Society (SDS) in the late 1960s.

In the Weathermen, she formed part of the radical Women's Militia and was charged with assaulting a police officer in Chicago during the 1969 "Days of Rage" riot. It was while she was free on $40,000 bail on the assault charge that the townhouse explosion occurred. She was last seen fleeing half-naked from the burning house.

Yesterday Wilkerson said, "Rather than putting all my energies and those of others into preparing for and enduring what could be a long and arduous trial, I want to begin above-ground political work around the people and issues which are my real priorities."

Those issues are not likely to be much different. In a statement on July 8, Wilkerson said her surrender "should not be taken to mean that my beliefs have changed or that social and political conditions have improved . . . It is 1980, but the conditions still exist which cause colonized peoples to fight for liberation here and in the rest of the world. These conditions cause me and others to continue to oppose the practices and principles of imperialism."