Rhodesia nationalist leader the Rev. Ndabaningi Sithole today called for a national unity conference of all African leaders and thirty-eight black groups to organize a campaign to force the Rhodesian government of Prime Minister Ian Smith to accept Anglo-American peace proposals.

Although the effort is designed to boost the slim chances of the Anglo-American plan for a new majority-rule government and independence next year, it is unlikely that Sithole's militant opporition will be interested in it. Sithole admitted today that none of the organizations had accepted his invitation to the conference, scheduled for Sept. 17.

(Spokesmen for two of Sithole's rival Rhodesian nationalist leaders, Bishop Abel Muzorewa and Joshua Nkomo, reacted coldly. Reuter reported, Nkomo's spokesman said, "We are not interested." An aide to Muzorewa said it is unlikely that his organization will attend.)

At a press conference in the capital, Sithole, an American-educated Methodist minister, denounced the prime minister's plans for an alternative "internal settlement" involving a broad-based government of both blacks and whites. Sithole was believed to have been a key figure in Smith's plans, and his announcement today could be a serious blow to the Rhodesian leader's hopes for his settlement plan.

It appears that Sithole, by calling together all African factions that might play a part in the "internal solution," is hoping to undercut Smith and force the government into accepting the Anglo-American proposals outlined here last week by British Foreign Secretary David Owen and Andrew Young, U.S. ambassador to the United Nations.

In an official statement, Sithole called on his rivals and followers to "join in national cooperatin to face the common political enemy, the enemy of the one-man, one-vote principle."

The appeal went out to Sithole's chief rivals - Nkomo and Robert Mugabe of the militant Patriotic Front and Muzorewa - and religious, labor, and professional groups, plus the tribal chiefs of Rhodesia. The list even included the Salvation Army, the National Football Association, the Girl Guides and the YMCA.

Sithole denied today that the conference plan was designed to put him in the key leadership posiiton inside Rhodesia. He promised that delegates - five from each group - would be free to elect any chairman. But many observers saw the move as an attempt to regain the leadership of the country's 6.3 million blacks.

Sithole's intense activities are being largely directed by a new figure in black politics here, a white American businessman from California named Neville Romain, who describes himself as Sithole's "broker." Romain moved into the Sithole camp last month and has been organizing all aspects of Sithole's return to Rhodesian politics.

Support of Nkomo's and Mugabe's Patriotic Front may not be necessary for Sithole to succeed because the militants are based outside the country and are not a strong factor in the "internal settlement," proposal.

The key question is whether the campaign will be supported by moderate Africans, particularly Bishop Muzorewa, who is believed to be supported by a majority of blacks inside Anglo-American plan, but he has also denounced Sithole.