As demonstrations and arrests begin to draw more attention to the segregationist policies of South Africa, TransAfrica's Capitol Hill office is buzzing with new-found importance.

Telephones ring almost constantly and a photocopier clacks out press releases. TransAfrica, serving as mission control for the ongoing antiapartheid protests at the South African Embassy, is straining at the seams, and the young activists who staff the foreign policy lobby find themselves at the center of what some black leaders see as an emerging global civil rights movement.

"We really have so much to do to sensitize the American public toward Africa and the Caribbean and scarce resources with which to do it," said Randall Robinson, the lobby's executive director.

After toiling for years in relative obscurity, TransAfrica's young, black, well-educated staff members say they are eager to make the best of the lobby's new -- and perhaps transient -- prominence. Despite all the new demands, staff members say this is what they have been waiting for.

"I think adrenaline has been keeping us going at this point," said David Scott, 22, TransAfrica's legislative liaison for military and political affairs, who rides from Northeast Washington to the office each day on his 18-speed bicycle. "We've all been working very long hours and no one's seen a weekend."

For weeks, Robinson has lived a hectic existence revolving around meetings, press conferences, demonstrations, arrests and arraignments. Robinson, 43, who was one of the first to be arrested at the embassy, concedes that "the pace is different" around the TransAfrica office, but maintains that nothing has substantially changed for the seven-year-old organization.

"This is perhaps the most media-covered campaign we have conducted, raising the profile of the institution," Robinson said recently. "But we certainly will continue to lobby the Congress, present testimony and marshal support for legislation we favor and oppose legislation that we don't."

Late last week, Cecelie Counts, a TransAfrica research fellow, was on duty at the South African Embassy, standing nearby as a marker to remind marchers they could not venture closer without threat of arrest.

A fixture at the demonstrations, Counts, 29, is a recent graduate of Harvard Law School and founder of the Southern African Support Project, a local group that raises funds and supplies for the people of southern Africa. She works mostly for TransAfrica Forum, the lobby's education and research arm, which publishes journals and papers.

"It's hard for people like us, people who are the affirmative-action generation who missed the civil rights movement but benefited from it," said Counts, a native of East Orange, N.J. "I think people like us have a greater obligation to make a contribution to our people."

Back at TransAfrica's office, David Scott is on the telephone, and he is annoyed. President Reagan had just claimed credit for Friday's release of 11 jailed black South African labor leaders. Scott, a New Yorker of Jamaican heritage who graduated from Tufts University in economics and political science, believes the credit belongs to the demonstrators. "Is there no justice?" he asked a caller.

Founded in 1977, TransAfrica seeks to influence Congress and the executive branch into making "progressive and enlightened" policies toward the black Third World, the group says. And during those years, TransAfrica -- which claims about 10,000 members nationwide who pay to support the group -- has established a solid reputation for professionalism and integrity, particularly among members of the Congressional Black Caucus that helped create it.

TransAfrica does have its critics, who say the group is prone to leftist politics that win as many enemies as friends.

Robinson acknowledges it is difficult to raise the lobby's estimated $400,000-a-year annual budget, but he says that has more to do with the lobby's policy not to accept funds from any corporation doing business in South Africa than its image.

Robinson, brother of television broadcaster Max Robinson, says TransAfrica salaries are modest, and added that as executive director he makes less than $35,000 a year. He jokes that his salary is "pulling down the curve" of his fellow members of the Harvard Law School class of 1970.

"I'm a lot more concerned about the condition of black people in the world than I am about material achievement," said Robinson, before he drove off to the day's demonstration in his 1976 Chevrolet. "What counts in the last analysis is a different kind of fulfillment that really can't be quantified."