The South African government has shot itself "in both feet" by placing restrictions on the media and detaining antiapartheid forces, Assistant Secretary of State Chester A. Crocker said yesterday.

"There are more than enough means of security laws and repression existing in South Africa's current laws without all these additional measures, which simply get in the way of any possibility for dialogue and polarize the issue further," Crocker said on "This Week With David Brinkley" (ABC, WJLA).

Crocker said the measures "will not address the basic grievances and the basic problems. What they will do is to put a buffer between our eyes and what is going on in the black townships," an action that he said "doesn't really fool anybody."

Although the United States condemns the Pretoria government's actions, Crocker said the Reagan administration will continue its policy of "constructive engagement" in an attempt to foster change in South Africa. Under apartheid, 5 million South African whites deny voting rights to 24 million black compatriots.

Referring to efforts by the government and U.S. companies with interests in South Africa to effect change there, Crocker said: "I think if more Americans knew what our firms are doing, . . . they would be proud of the role that we are playing in that country."

Crocker said the administration will "firmly oppose" legislation passed last week by the House that would require total U.S. disinvestment from South Africa and impose a boycott against that country except for key minerals.

The United States must avoid measures that "assure that we bring destruction to South Africa in the form of collective punishment on all the South African people," Crocker said.

He said the United States will continue to urge negotiations between blacks and whites to create a new political system in the country.

Such negotiations, Crocker said, should include the African National Congress, the outlawed black nationalist organization based in Zambia.

Earlier in the broadcast, Louis Nel, South Africa's deputy minister of information, said Pretoria will not negotiate with the ANC unless it halts its violent campaign to overthrow the South African government and purges itself of its communist element.

Nel said radical groups such as the ANC killed 34 of the 56 blacks reported to have died in South Africa last week. Nel said such groups "are not interested in reform. They're not interested in a new constitution. They are not interested in power-sharing. They want a revolution."

Randall Robinson, executive director of TransAfrica, said the blacks who killed other blacks are conservatives instructed and armed by the white police.

Thabo Mbeki, a spokesman for the ANC who also appeared on the show, said apartheid is the real cause of the violence. "So long as you have apartheid, so long will you have violence," he said. "That is a basic, fundamental thing." Mbeki also said that the ANC began arming itself after 48 years of unsuccessful, peaceful resistance.

Mbeki acknowledged the presence of communists in the ANC, but said the group is committed to democracy. "The ANC is not a communist plot," he said. "Within the ANC you have communists, you have people that have got Christian democratic views, you've got people who are Muslim, . . . Christian -- and all of them make together this formation, this alliance."

Sen. Jesse Helms (R-N.C.), appearing on "Meet the Press" (NBC, WRC) yesterday, also questioned the ANC's makeup, calling it a "communist-front organization."

"Apartheid is dead. Let's have the funeral for that," Helms said. "What we're talking about is the future of blacks and whites in Africa. Will they have a future under a communist regime . . . or a future under a free system?"

Helms said that blacks, "orchestrated by communists," are killing other blacks. "This is a point not made clear in the news media in this country," he added.