One year after three American black leaders were ordered from the South African Embassy here and refused to leave, the burgeoning antiapartheid movement launched by their Thanksgiving Eve arrests is digging in for more street protests, harder lobbying in Congress and a stepped-up campaign against corporate investments in the racially torn nation.

Organizers of the Free South Africa Movement, the civil rights coalition established after those initial arrests, will mark the first anniversary today with a noon march to the embassy and a new batch of arrests to add to the 4,000-plus that have already occurred there and around the nation.

The demonstration will be a celebration of sorts for the group, largely credited with putting South Africa's apartheid system of racial segregation on the front burner of American consciousness. But protest leaders say there will be no resting on their laurels.

"This is going to be a day of recommitment," said Randall Robinson, national coordinator of the antiapartheid movement and one of the three protesters -- along with D.C. Del. Walter Fauntroy and U.S. Civil Rights Commissioner Mary Frances Berry -- whose embassy arrests began the campaign.

Supporters, he said, are preparing to give Nobel laureate Bishop Desmund Tutu a "Freedom Letter" signed by 1 million Americans who want apartheid dismantled. He and other coalition leaders had hoped to travel to South Africa to present the letter in person, but because South African officials have yet to approve their visas the group is now trying to arrange for Tutu to come here.

There also will be a renewed push in Congress for tougher economic sanctions against the white minority-ruled nation, a move that stalled in September when President Reagan signed an executive order containing milder sanctions.

The president, who favors a course of quiet diplomacy to bring about racial reforms in South Africa, averted a showdown with Congress. But Robinson said he expects the issue "to resuscitate" during next year's Senate races.

In addition, protest leaders say they will announce soon the name of a multinational company that will figure in an expanded antiapartheid campaign against firms that do business with South Africa. The firm is expected to become the target for picketing and civil disobedience.

At least eight states and 32 local governments have divested their business holdings in South Africa, and antiapartheid activists said they hope to increase such actions in the year ahead.

Today's "Day of Recommitment" gathering is attracting support from several quarters. Comedian Bill Cosby, for one, has quietly provided his public relations firm to help promote the anniversary event.

The demonstration will get under way at noon with a march from 23rd and P streets NW up Massachusetts Avenue to the embassy. About 6,000 supporters are expected to join the demonstration, according to a parade permit application filed with D.C. police, and more than 100 arrests are expected.

Those scheduled to attend a rally near the embassy include Fauntroy, Mayor Marion Barry, Gary (Ind.) Mayor Richard Hatcher, National Organization for Women President Eleanor Smeal, tennis star Arthur Ashe, United Mine Workers of America President Richard Trumka and Episcopal Bishop John Walker.

In assessing progress after a year, Robinson said the antiapartheid movement had succeeded in placing the issue "in the middle of the American public affairs debate." And he said it is making progress on a second goal: getting the U.S. government to adopt a tougher stance and stiffer economic sanctions against Pretoria.

Robinson said he believes that until U.S. policy changes, there will be little hope of progress on the movement's chief objective, getting South Africa's white leaders "to come to the table and negotiate toward the dismantling of apartheid and the granting of one-person, one-vote in a democratic state."

Trumka, whose union has filed a court challenge to the importation of South African "slave labor" coal, said labor leaders are prepared to lend their support to an escalated campaign against the apartheid system.

"I think you'll see us continue with the consumer boycotts of companies that are propping up South Africa," he said.

Union members have already pitched in to work for sanctions legislation and to help collect signatures for the "Freedom Letter" to Tutu.

Trumka said he just returned from visiting several mines and universities around the country and found that miners and students raised the issue of apartheid wherever he went.

Support for the antiapartheid movement exists, according to Robinson, because Americans now have a sense of what is going on in South Africa, and because the issue of race relations is one the United States has struggled with firsthand.

"A response to apartheid is a response to 30 million black Americans," he said.

Robinson, executive director of the foreign policy lobby group TransAfrica, said the organization's nine-member staff has had help from volunteers in handling the ongoing embassy protests.

Mentioning a man who brings fruit and candy to the picket line and the District church that has donated its bus to drive demonstrators, Robinson said the movement "belongs to all who have contributed and to whom we all owe a great debt."