PRETORIA, SOUTH AFRICA, JUNE 27 -- The powerful Dutch Reformed Church, to which most of South Africa's politically dominant white Afrikaners belong, split today when extreme conservatives decided to break away and form their own new church committed to preserving strict racial segregation.

The decision was made at a rally here of 2,000 conservatives who came from all parts of the country to protest what they regard as a shift toward more liberal doctrine by the Dutch Reformed Church, which in the past has given vital theological sanction to the apartheid policy of white domination.

After a five-hour meeting behind closed doors, the extremists ordered a special Committee of Dissatisfied Members to found a new church that will uphold the principles of rigid apartheid and whites-only membership.

Spokesman Willem Lubbe said after the meeting that the new church would be called the Afrikaans Reformed Church. He described it as "a church of Christians among white Afrikaners" and said it would hold its first services here on Sunday.

Among other things, Lubbe said, the new church would forbid marriages between whites and nonwhites, which the government recently legalized with the approval of the Dutch Reformed Church after years of prohibition.

The schism in the church follows a political split that rent the once solidly united Afrikaner community in 1983, when Andries P. Treurnicht broke away from the ruling National Party in protest against reforms to the apartheid system introduced by President Pieter W. Botha.

Soon after Treurnicht's break, the influential Broederbond secret society, an inner core of elitist decision-makers who controlled almost every aspect of Afrikaner life, also split. Former chairman Carel Boshoff, another theologian of the church, formed a rival far-right body called the Afrikaner Volkswag.

The church split comes as differences over the morality and political practicability of apartheid continue to shake Afrikaner nationalism.

Such a split in the religious ranks of the devout Afrikaner people is potentially the most disruptive of all. The influence of the Dutch Reformed Church is enormous. More than 70 percent of Afrikaners belong to it, including most Cabinet ministers and all of the prime ministers and presidents since the country was founded in 1910.

The Dutch Reformed Church's religious teachings over decades helped to form the ideology of apartheid. They provided the theological justification for regarding the white Afrikaners as a distinctive volk with a God-given right to exist as a separate nation with their own homeland in which blacks would have no citizenship rights.

The church itself was segregated between the white or "mother" church and separate "daughter" churches for blacks, the mixed-race coloreds and a small number of Asians converted to the Dutch Reformed faith by missionaries.

This theological underpinning of apartheid has been coming under increasing pressure in recent years, with colored theologians headed by Allan Boesak, a leading antiapartheid activist, and a growing number of white dissenters challenging it.

The pressures intensified when the World Alliance of Reformed Churches declared apartheid to be a heresy at its conference in Ottawa in 1982, at which it also elected Boesak as its president.

A year ago, the colored branch of the church, which Boesak heads, called on the "mother" church to renounce apartheid, threatening to sever relations if it did not do so.

While stopping short of actually branding apartheid a heresy, a church policy adopted at a synod last October conceded that apartheid was morally wrong and that it had done grievous harm to the people of South Africa. It declared that the church should be open to all races.

Last Monday, the church's General Synodal Commission met with Lubbe's committee to consider the group's criticisms, which amounted to a demand for the revocation of the "Church and Society" reforms.

Members of the commission reportedly tried to mollify the dissidents, suggesting that the decision to become an "open" church would be largely token and that blacks would be admitted only under exceptional circumstances. But the extremists remained adamant that the decision violated long-accepted theological principles.