Leaders of a Commonwealth effort to help negotiate peaceful change in South Africa said today that the West has only "the next few weeks, a month or two," to try to prevent all-out racial war in South Africa, and urged the United States, Britain and West Germany to impose widespread economic sanctions against Pretoria.

Unless South Africa's largest western trading partners, investors and creditors move quickly in their "own enlightened self-interest," the Commonwealth leaders said, the inevitable result will be the emergence of a "radical" black government that "will destroy western interests absolutely" and will likely owe its allegiance to the Soviet Union.

Among the immediate measures they recommended were the suspension of all air links to and from South Africa; a ban on all purchases of South African "bulk commodities," including agricultural goods, raw materials and industrial products, and a further tightening of existing financial restrictions.

Although many of the 49 Commonwealth nations already have taken substantial measures against Pretoria, Britain, the most prominent member of the organization that includes most of its former colonies, has resisted, on the grounds that sanctions are not effective. Without Britain, said former Australian prime minister Malcolm Fraser, the "trade weight" of the others "is not sufficient to achieve change."

In addition to Britain, Fraser said, "it has to be the Americans. It has to be the [West] Germans, also." About three-quarters of all foreign private investment, and the majority of foreign companies represented in South Africa come from those three countries. Nearly half the investment is from Britain alone. Together the three hold the major portion of Pretoria's foreign debt; only Japan has a closely equivalent level of trade with South Africa.

Fraser and former Nigerian head of state Olesegun Obasanjo are co-chairmen of the seven-member "Eminent Persons Group" appointed by the Commonwealth last October "to foster negotiations" leading to democratic change in South Africa.

After spending six months talking to all sides in South Africa and neighboring black African states, the group has reported back that the Pretoria government "is not interested in negotiating" and that a bloodbath of catastrophic proportions may be unavoidable.

Publication of the group's report today was marked by a news conference held here by Fraser and Obasanjo. As if to underscore its conclusions, the conference was interrupted when an aide handed Fraser a note saying that, moments before, the South African government had announced the imposition of an unprecedented nationwide state of emergency.

Although the chairmen said that they now are convinced Pretoria never seriously wanted to disuss substantive change and engaged in a propaganda exercise in meeting with them, they said they remained hopeful until about five weeks ago.

At that point, Obasanjo said, "something had snapped." In a meeting with Cabinet officials, he said, South African Foreign Minister R. F. Botha "lost his cool" over what the group felt had been a painstakingly worked out, key issue. Botha, he said, informed them that what they felt was a previous agreement by all sides on "suspension" of violence was no longer acceptable. The government then insisted that all black organizations "renounce" violence "for all time" before it would begin negotiations.

"After consultation, and in our own judgment," Obasanjo said, the group concluded that it was an "unreasonable and unrealistic" demand to make of "a people who have no political rights and little economic rights."

British Prime Minister Margaret Thatcher gave a cautious and noncommittal response to the report today, telling Parliament she would "consider [it] carefully, with our partners, before reaching conclusions."

But the group's chairmen, along with Commonwealth Secretary General Shidrath Ramphal, warned that Thatcher would risk fracturing the organization if she rejected the report's recommendations.

"Mrs. Thatcher has a sense of history," said Obasanjo. "She won't want to be the British leader who broke up the Commonwealth."

In addition to the chairmen, the group included British Conservative former chancellor of the Exchequer Lord Barber; Archbishop Ted Scott of Canada; Dame Nita Barrow of Barbados, copresident of the World Council of Churches; Swaran Singh, former foreign minister of India, and John Malecela, former foreign minister of Tanzania.

South Africa's actions, Fraser said, are not those of "a government trying to create a climate in which to negotiate with legitimate leaders" in the black opposition. As a result, he said, "sanctions are the only alternative" for concerned western governments.

Pretoria is "terrified" of them, he said, but its conviction that it will be protected from them by the United States and Britain, in particular, has allowed it to defer change.

Without the final effort of sanctions, Fraser said, "the descent into greater violence could be quite rapid." Black leaders and militants, particularly those in the banned African National Congress, which the report said enjoys "incontestable" support among South African blacks, already will take the report's conclusions as "confirmation that the government is not serious" and conclude that "all that is left for them is to fight it out."