RUSTENBURG, SOUTH AFRICA, NOV. 9 -- An ecumenical conference of Christian churches joined today in issuing an unprecedented, sweeping condemnation of the churches' "heretical part" in South Africa's apartheid policy of racial segregation. But the powerful white Dutch Reformed Church withdrew its support from parts of the declaration at the last moment.

Winding up the country's largest ecumenical conference in 30 years, 230 delegates from 80 churches and 40 other church-related groups agreed on "the unequivocal rejection of apartheid as a sin" and confessed "our heretical part in the policy of apartheid which has led to such extreme suffering for so many in our land."

"We denounce apartheid in its intention, its implementation and its consequences as an evil policy," the so-called Rustenburg Declaration said. "The practice and defense of apartheid as though it were biblically and theologically legitimated is an act of disobedience to God, a denial of the Gospel of Jesus Christ and a sin against our unity in the Holy Spirit."

Conference sources said it was the strongest denunciation in South African history of the churches' role in giving apartheid political support and theological justification.

But even before the final draft was completed, leaders of the white branch of the Dutch Reformed Church issued a statement dissassociating themselves from large portions of the declaration.

When other delegates became aware of the statement, their reaction was bitter, spoiling hopes for any real reconciliation of the anti-apartheid churches with the white Dutch Reformed Church and undermining efforts toward unifying it with its segregated branches for blacks, mixed-race Coloreds and Indians.

Sam Buti, head of the black Dutch Reformed Church branch, accused the white church's leaders of backtracking under pressure from ultra-conservative whites. Russel Botman, the deputy leader of the Colored Dutch Reformed Church, warned that the statement "could be devastating" to the unity movement within the church.

Anglican Archbishop Desmond Tutu was more cautious, saying much would depend on the "nitty-gritty" of what the white Dutch Reformed Church was rejecting in the Rustenburg Declaration. But if it included "crucial elements" of the statement, he warned, that would be "disastrous."

Tutu said he and others had "put ourselves on the line" in accepting the sincerity of the confessions of sin and guilt made before the conference by Dutch Reformed theologian Willie Jonker and the church's moderator, Pieter Potgieter.

The conference's co-chairmen, Frank Chikane and Louw Alberts, sought to minimize the damage caused by the white Dutch Reformed delegation's action. "The good news is that we came together and that we talked to one another," said Chikane.

Potgieter insisted to reporters outside the conference hall that he and Jonker were not retreating from their confession of apartheid as a sin. But he said his delegation did not know what the declaration's "unequivocal" rejection of apartheid meant. The delegation also had problems, he added, with the declaration's condemnation of military conscription and its call for a "democratic elective process based on one person, one vote," a constitutional assembly and an interim government.

"These are matters for politicians, not the church," Potgieter said.

In a vain attempt to address the white Dutch Reformed delegates' objections, the conference eliminated any mention of conscription and watered down references to the constitutional assembly and interim government, both demands being made by the African National Congress.

Several delegates said they believe Potgieter and Jonker were acting to protect themselves from right-wing reaction to their confessions.

The leader of the white Conservative Party, Andries Treurnicht, issued a statement distancing himself from their confessions. Newspapers reported today that former president Pieter W. Botha had telephoned Jonker to berate him for his action.

Some Dutch Reformed delegates estimated that as many as one-third of the white church's 1.5 million members, most of whom are Afrikaners, belong to the Conservative Party. These members, they said, might be spurred by the confessions made here into joining 30,000 others, including 70 ministers, who left in 1986 to form a separate all-white church.