A leader of the banned African National Congress today declared that protests by South African blacks to mark the 10th anniversary of the Soweto riots were a "resounding success," despite Pretoria's information clampdown.

Addressing a United Nations conference on sanctions against South Africa, ANC leader Oliver Tambo, whose group is waging a guerrilla war to overthrow the apartheid system, also attacked western countries for allegedly shielding the South African government. The five-day conference is being boycotted by the United States, Britain, and West Germany, South Africa's main western trading partners.

Tambo, who is normally based in Lusaka, Zambia, said that the "screen on public information" imposed by Pretoria had made it difficult to establish what exactly happened during the protests yesterday across South Africa. He said that some unconfirmed reports suggested that there could have been a "massive slaughter because people would have gone ahead with rallies and demonstrations."

The South African government, which has imposed tight restrictions on independent reporting, has confirmed the deaths of 11 persons during the protests. Tambo did not reveal the source of his information and could not be reached later for comment.

The ANC representative described yesterday's protests as a "resounding success, the greatest national strike in the history of South Africa, an act of defiance before South African military might."

Warning that time was running out in South Africa "if it hasn't run out already," Tambo said that a bloodbath was inevitable unless the international community took immediate action. He said that countries like the United States which refused to impose sanctions against Pretoria were "co-conspirators in a crime" and "allies of a murderous regime."

Third World leaders, meanwhile, stepped up their pressure on European Community countries to take action against South Africa. Yesterday, west European foreign ministers failed to reach agreement on new sanctions, postponing a decision until the community's summit in The Hague next week.

French and Irish ministers told the U.N. conference that their governments would press for action against South Africa. Britain and West Germany oppose sanctions, saying they are unlikely to influence the Pretoria regime.