JOHANNESBURG, JAN. 8 -- South Africa today moved a step closer to negotiations over a new constitution as African National Congress leader Nelson Mandela proposed that a conference be convened of all the country's political parties to lay the groundwork for drawing up a democratic document.

ANC leaders said the conference, which would serve as an interim body, was designed to bridge the gap between the white-minority government and its black opponents over who should draw up the new constitution and who should rule South Africa while its future was being negotiated.

A senior government official tonight welcomed the ANC proposal. Minister of Constitutional Development Gerrit Viljoen, a key architect of Pretoria's political changes, said in a statement that such a conference would help ensure the legitimacy of the negotiation process.

Neither side offered a date for a conference, but both agreed one should occur as soon as possible. "What we emphasize is the urgency of the matter," Mandela told a press conference at the ANC's temporary headquarters here.

Until now, negotiations have been limited to President Frederik W. de Klerk's government and the ANC, which is the most prominent black opposition group. These "talks about talks" have resulted in agreements on the release of political prisoners, the return of exiles to South Africa and the suspension by the ANC of its armed struggle against white rule.

There is also general agreement that a new constitution, whose adoption would mark the death of the apartheid system of white domination, should be democratic and non-racial and should include a bill of rights and provisions for an independent judiciary.

But the two sides remain far apart on who should draw up a new constitution. The ANC has insisted that it be written by a constituent assembly chosen in a nonracial election, and that an interim government should hold power during that period. ANC officials contend the present government lacks legitimacy among blacks and should not serve as "both a player and a referee" in the process.

"Quite clearly, this process of transition away from apartheid cannot be supervised by an apartheid institution, which is precisely what the present government is," the ANC said in an executive statement read by Mandela.

Government officials have insisted that until a new document is drawn up and approved, they must abide by the present constitution, which makes no provison for an interim government. They have also ruled out a constituent assembly chosen on a non-racial basis, saying that because blacks are a majority in South Africa, such an arrangement could give black organizations too much power in writing a new charter.

Mandela insisted that a so-called "all-party congress" could not serve as a non-elected substitute for a constituent assembly or write a new constitution itself. But he said it could set out "broad principles" for such a document, determine the makeup of the body that would write it and establish an interim government until a new constitution takes effect.

He even suggested that the congress could turn into a constitution-writing body if it first submitted to a free, nonracial election.

Such a congress could expand the field of participants in negotiations from the ANC and the ruling National Party to include such ideologically varied participants as the whites-only Conservative Party, which seeks to preserve apartheid; Zulu Chief Mangosuthu Buthelezi's Inkatha Freedom Party, which is involved in bloody feuding with the ANC; and the Pan-Africanist Congress, whose informal slogan is "one settler, one bullet."

Mandela said he was prepared for all of these groups to participate, and said he expected the South African Communist Party to attend as a separate participant from the ANC, its longtime political partner. But Mandela said how many votes each party would wield at the congress would have to be worked out.

"An all-party congress means exactly that. . . . It would be a mistake for the ANC and the government to think they are the only parties to be involved in the process. . . . We are not the only actors," Mandela said.

In responding to the proposal, Buthelezi expressed suspicion that the congress would be designed to help the ANC take power, while the Conservatives said they would not attend because the meeting would not safeguard "white rights." But analysts here said that if the ANC and the government agreed on such a session, it would be hard for any major political party to stay away.

The ANC proposal was the most conciliatory section of a generally tough and unyielding "state of the nation" report commemorating the organization's 79th anniversary.