DURBAN, SOUTH AFRICA, AUG. 31 -- President Frederik W. de Klerk today urged his whites-only National Party, the architect of South Africa's apartheid system, to open its membership ranks to all races.

De Klerk said he was making the watershed proposal to ensure that the party remained a viable political force in the new society he is trying to negotiate with South Africa's black leaders. Responding to de Klerk's call, a convention of party officials representing the Natal provincial branch voted unanimously to support a change in the party's constitution to allow blacks to join.

The same proposal will now go before three more provincial conventions of the party, and if they all endorse it, as seems certain, the party that devised the apartheid system of total physical and political segregation will become multi-racial by December.

In addition to trying to recruit black members directly, party leaders at the convention here said they would seek to form alliances with moderate black political organizations that share their views on opposing socialism and preserving cultural values.

Although party officials were reluctant to speculate on which black movements might join such an alliance, Minister of Education Stoffel van der Merwe, who also heads the party's information services, hinted in an interview that Zulu Chief Mangosuthu Buthelezi's Inkatha Freedom Party and other tribally rooted organizations might be candidates.

He implied that the National Party had in mind emulating the cluster of ethnic parties that joined forces to form the Democratic Turnhalle Alliance in Namibia, which was founded and led by a white Afrikaner, Dirk Mudge.

National Party members were impressed by the alliance's ability to win more than 28 percent of the vote in Namibia's independence elections last November and deny the radical South West Africa People's Organization a free hand in drafting the country's constitution.

De Klerk clearly hopes that if he can form a similar alliance, his party may still be able to share in power after majority rule comes in a country where blacks outnumber whites 5 to 1. This possibility would be strengthened if, as some predict, the major black nationalist movement, the African National Congress, were to split into its radical and moderate wings after majority rule was achieved.

Presenting his proposal to the party convention, de Klerk said there would be "an inevitable realignment in the party-political sphere in the new South Africa which we are building." The National Party should therefore lay the basis now for future cooperation with other parties, he said.

"The National Party will work for alliances, or a broad political movement, which unites those who think alike in respect of common goals on the basis of shared convictions," de Klerk added.

Many party members at the convention spoke with feeling about the need to uphold their Christian values and negotiate a fair settlement with South Africa's blacks.

De Klerk captured the mood when he said in his keynote address that opening the party to all races meant that "racism and racial discrimination in South Africa are now over forever."

"The step we have taken today is an important one on the way to creating a new and just South Africa," de Klerk said. "We are doing what we are doing, we are taking the initiatives we are taking and will continue to take to their logical conclusion, not just for the sake of our country's future, not for political expediency, but because it is right and just -- and because only by doing what is right and just can this country have a sound future.

"I cannot accept that the Afrikaners, the whites, can seek to build their future on injustice toward others," the president added.

Explaining for the first time why he had made a dramatic policy about-face since becoming president a year ago, de Klerk said that in the past the National Party government had sought to create a just partition between blacks and whites.

Previous leaders had pursued that ideal, which they believed to be just, but it gradually became apparent that the ideal was unattainable, he said. At a party congress in August 1986 the leadership had finally been compelled to acknowledge that the goal could not be attained.

Once that was recognized, de Klerk said, the truth had to be faced that white rule could only be maintained by oppressing the black majority, and that to continue with it would be "a formula for disaster."

Attacking the far-right Conservative Party, which he said would seek to profit from the National Party's decision to integrate, de Klerk accused it of irresponsibility in continuing to pretend that the goal of a just partition is attainable.

"To cling to a policy that you know cannot work is dishonest," de Klerk said. "The Conservatives claim to be protecting the interests of the whites, but what they are doing is a recipe for revolution that will destroy the whites."

In Pretoria tonight, Conservative Party leader Andries P. Treurnicht described the National Party's move as "a political surrender" and "a tragic sacrifice of the principle of self-determination of nations."

The ANC gave the National Party's decision a lukewarm welcome, describing it laconically as "better late than never."

Pallo Jordan, the ANC's information chief, said, "I'm sure there are some blacks who will join, but I can't imagine why."