Celebrating their first major legislative victory since they began their protest against racial discrimination 6 1/2 months ago, antiapartheid demonstrators gathered at the South African Embassy again yesterday and cheered House passage of proposed investment sanctions against the white minority-ruled nation.

"We can say now with some confidence that this Congress will enact major sanctions" against South Africa," said Randall Robinson, coordinator of the embassy demonstrations.

He credited the protests, which have resulted in more than 3,000 arrests nationwide, with spurring congressional interest in apartheid sanctions, and praised "this unbroken string of people who have been arrested on this site every day since Nov. 21."

Robinson called House and Senate support for South Africa sanctions encouraging and said votes cast in the House Wednesday indicate that members of Congress would be prepared to enact even tougher measures if the South African government does not respond to this year's legislation.

He said the embassy protests will continue into the fall until the Senate has approved, and the president has signed, the antiapartheid sanctions.

A spokesman for the South African Embassy, reading from a statement released by the government's deputy minister of foreign affairs, said yesterday that final approval of such sanctions would affect future employment in the country and "lead to immeasurable suffering, unemployment and starvation in Southern Africa -- a situation which would be totally beyond the control of the United States."

Three counterprotesters, who have begun once-a-week pro-South Africa demonstrations across the street from the antiapartheid actions, said yesterday that the sanction would help "black Marxists" and threaten the survival of the United States.

"If we support the sanctions, we'll have totalitarianism," said Mark Wesley Hart, president of American Citizens for South Africa, a group affiliated with the Rev. Carl McIntire, president of the International Council of Christian Churches.

The two groups traded chants across Massachusetts Avenue NW, with the three pro-South Africa demonstrators accusing the apartheid protesters of being "tools of the communists," and about 20 antiapartheid demonstrators all but drowning them out with their own calls to "Free South Africa."

Les Arthur, a 27-year-old Howard University graduate student, was the only one arrested at yesterday's protest. He said it was the first time he had ever joined in an act of civil disobedience.

"We all need to get involved," said Arthur. For such a cause, he told the other apartheid protesters, getting arrested "is really an honor."