Helicopters whirred overhead at the giant South African fuel plant, dropping leaflets threatening 6,500 black workers with firing if they didn't end their strike.

Then, huge armored vehicles rolled in, as Army troops and police armed with tear gas broke up a gathering of union members, arresting several. Then the loudspeakers blared the news that all the workers were dismissed.

"We are living in a world of rubber bullets and tear smoke," said Calvin Makgeleng, one of three South African labor officials who yesterday offered that description of events in South Africa during the Nov. 5-6 strikes that have prompted the current wave of "Free South Africa" protests here..

Demonstrators in recent weeks have demanded the freeing of 21 South African labor union leaders jailed without charges during last month's walkouts by students and hundreds of thousands of workers, the largest political strike in that nation's history.

But Makgeleng and the other officials who held a press conference here as part of an effort to enlist American support, said the jailing is only part of a government effort of "union bashing" to suppress the nation's fledgling black labor movement.

"The state is sending cops into the factories, to have factory workers give evidence in court against union leaders . The state is clamping down on the union leadership," said Makgeleng, chairman of the Chemical Workers Industrial Union in the Transvaal province.

The firing of 6,500 workers from the state-controlled Sasol Ltd. coal conversion plant is part of a government effort to subvert unionization, said Pat Horn, an organizer for the Paper, Wood and Allied Workers Union who was banned from the country for five years in connection with her union activities.

The firings came during negotiations in which the chemical workers' union was attempting to win a first contract, Makgeleng said. The firings, coupled with the arrests of the nation's leading black labor officials, sends a signal to both South African and American firms "that the state is taking the lead . . . in intimidating union members," he said.

Black South Africans, banned from unions until 1980, are "fighting for living wages," said Horn, who said only automobile workers earn more than subsistence wages. "In the homelands," she said, referring to rural black townships plagued by high unemployment, "There are starvation wages . . . and misery and poverty is much higher" than the rest of the country.

The AFL-CIO has strongly supported the South African unions and has urged stronger action by the Reagan administration to pressure the government to end the apartheid system. The labor federation has said the low wages paid by governments such as South Africa translate into lost jobs here as multinational corporations are drawn overseas to take advantage of cheap labor.

South Africa's ambassador to the United States, Bernardus G. Fourie, said that "the South African government is in favor of trade unionism. The fact that the government was responsible for introducing a trade union system . . . which is as modern as any system in the world should be proof of this fact."

He said the labor officials were being detained for investigation of possible violations of the Internal Security Act, "which relates to activities causing a breakdown of law and order . . . " and would be released or charged with crimes as soon as the investigation is completed.