Organizations representing the employers of 80 percent of workers in South Africa's commerce, industry and mining urged President Pieter W. Botha today to give "visible expression" to the promises of reform he made six weeks ago.

The joint statement by six business associations was the second move by South African business leaders this week pressing the government to implement reforms to counter the growing campaign in the United States for divestiture, or ending U.S. investment here.

Yesterday, several leading businessmen called for reform in speeches at a meeting of the South Africa Foundation, a normally low-key organization that lobbies for closer business ties between South Africa and the rest of the world.

Both developments came as the Reagan administration cast its first vote against South Africa in the United Nations Security Council, supporting an African bloc resolution that condemned the killing of demonstrators in the Crossroads shantytown outside Cape Town and the arrest of black political opponents on allegations of treason.

The South African business community traditionally has been critical of the country's system of racial segregation called apartheid, especially those aspects of it inhibiting the free functioning of the labor market and preventing blacks from acquiring skills. But until now, it generally has been reluctant to express these criticisms openly.

The government has responded to this open criticism with mild irritation. The minister in charge of black affairs, Gerrit N. Viljoen, told representatives of the six business groups yesterday that he appreciated their concern, but that South Africa should implement reforms for domestic, not external, reasons.

Constitutional Affairs Minister Chris Heunis was more forthright. Businessmen should not call for black political participation without understanding the implications of what they were advocating, he told a public meeting last night.

Foreign Minister Roelof F. (Pik) Botha, in speeches Tuesday and again yesterday, warned South Africans not to underestimate the divestiture campaign or the possible consequences of actions by the U.N. Security Council. He warned that "elements" in the United Nations wanted to promote violence through an economic recession in South Africa so that the Security Council could declare the situation a threat to world peace and enforce mandatory sanctions.

The six business groups had committed themselves in January, during a visit to South Africa by Sen. Edward M. Kennedy (D-Mass.), to full political participation and citizenship for all South Africans.

They said today that they "are keenly aware of the positive impact which visible progress along this road is likely to have on overseas opinion and especially on the current disinvestment debate in the United States."