On the 10th anniversary of the Soweto uprising of 1976, millions of blacks in South Africa conducted a telling protest against apartheid. For the most part it was not a loud or public protest, and certainly not the violent one that the regime had predicted and that, by predicting, it had used to rationalize its assumption of harsh new police powers. It was, however, a significant and effective protest in which the people withheld for the day their labor and purchasing power, which together make up the bulwark of white rule.

The police may retain indefinitely a great advantage in physical power, and they can prevent the black majority from holding mass demonstrations or mounting substantial armed attacks. But the police cannot make blacks work or shop in a normal way. This is the potential that South Africa's blacks mobilized impressively when the government attempted to deny them their day of mourning and resolve.

Like external sanctions, internal sanctions put economic pressure on apartheid, but in circumstances of the blacks' own choosing and with their awareness of and consent to the likely costs. Neither sort of sanctions is easy to organize: the Western governments that do open business with South Africa and the African governments that do quiet business have their economic and other reasons for continuing it, while the people of South Africa must, of course, live and eat. Internal sanctions, nonetheless, seem to fit a gathering mood of self-liberation. A desire to show solidarity will move many in the West to keep trying to extend external sanctions. The main arena, however, remains South Africa.

The Reagan administration finds itself on the defensive in a grinding domestic argument over sanctions. The argument arises, or its bitterness flows, because the president has failed to project a clear and feasible American role in the dismantling of apartheid. That role cannot be to take the lead role in forcing the South African government to change: South Africans will have to apply most of the pressures that will be brought to bear.

The American role should properly center on facilitating accommodation. A major roadblock here is the administration's own doubts about the African National Congress, a leading if not the leading black political organization. The administration urges Pretoria to open talks with the ANC, but, evidently apprehensive about the group's revolutionary thrust and Soviet ties, it hesitates itself to deal with it. This week in Washington the group's visiting secretary general was officially shunned. The message -- the dismally wrong message -- is that Pretoria can shun the ANC too.