Deepening resistance to South Africa's announced plans to cede two chunks of territory to the neighboring kingdom of Swaziland has brought black and white politicians here together in a rare common cause.

It is also forging a front between divided political movements among the blacks, who are putting aside differences to fight a deal that would strip 1 million blacks of their South African citizenship and make them Swazi nationals.

While Swaziland supports the proposal as a fulfillment of old claims, South Africa has offered no official explanation of why it is determined to give away land against the wishes of its inhabitants, particularly since historians say Swaziland's claim is of doubtful validity.

Most political commentators in South Africa assume it is partly to secure the friendship of a neighboring black state that is a member of the Organization of African Unity, and partly to further Pretoria's policy of giving tribal homelands independence as a facet of maintaining whites in power here.

The black leaders have won three provincial supreme court orders invalidating government proclamations to start the ceding process. A decisive appeal is due to be heard in the appeal court at Bloemfontein Aug. 18.

The joint political action began this week when the white opposition Progressive Federal Party teamed up with African leaders from the two affected regions, Chief Gatsha Buthelezi of KwaZulu and Enos Mabuza of Kangwane, for a series of protest rallies in defiance of a 1968 law prohibiting mixed-race politics.

Rallies have now been held in Durban, Pietermaritzburg and Johannesburg.

More than 2,000 people, two-thirds of them black, crowded the city hall here to overflowing Wednesday night, and black and white politicians sat together on the platform. It was the largest multiracial political rally in memory in this largest of South African cities.

Emotions ran high at the meeting, with raised-fist salutes and frequent shouts of amandla, the African nationalist slogan meaning "power." At the conclusion, blacks and whites stood together to sing the African anthem, "God Bless Africa," which has been officially adopted by the outlawed African National Congress.

The crowd spilled out, closed the street to traffic and danced and sang more African political songs to the departing speakers. South African police did not intervene.

The speakers included Buthelezi, Mabuza, a Progressive Federal Party leader, Ray Swart, and Bishop Desmond Tutu--a leading black who has voiced strong African nationalist criticism of the tribal homeland leaders for holding office within the government's segregationist system.

Tutu is secretary general of the South African Council of Churches. He and Buthelezi have been hostile for two years, but Wednesday night Buthelezi welcomed him as a "brother" and members of Buthelezi's Inkatha movement cheered when Tutu said, "Our South African citizenship is the one thing that is absolutely, totally nonnegotiable."

Buthelezi said he had also received a message of support from the African nationalist Azapo movement, and thanked the exiled African National Congress for deciding to oppose the land deal, too. It was a display of unity across what has been a deepening divide in black politics.

The government appears surprised by the reaction that its June 14 announcement of the land deal has provoked. It could face a problem with the king of Swaziland, 82-year-old Sobhuza II, if it tries to retract what may be a firm agreement with him. Nobody in Pretoria will say whether this is the case, but sources in the Swazi capital of Mbabane appear confident that South Africa has committed itself and will not withdraw.

The government has an alternative if it should lose the appeal Aug. 18. With its parliamentary majority, it can change the law at the next session, in February, then reissue its proclamations. But Buthelezi and Mabuza warned that government response could provoke bloodshed.

"Kangwane is a powder keg which could blow up at any moment," said Mabuza.

Sobhuza says the two territories, Kangwane and the Ingwavuma region of KwaZulu, are part of the kingdom Swazi kings ruled in the 19th century.

They are estimated to include 3,000 square miles, about half the size of Hawaii. This would double the size of landlocked Swaziland, and Ingwavuma would give it access to the sea, with a potential harbor at Kosi Bay.

Both Buthelezi and Mabuza have been thwarting South Africa's tribal-homelands policy by refusing to accept independence for their regions, insisting on the right of their people to remain South Africans and be granted political rights here.

Giving the regions to Swaziland would have the same political result as if the homelands were declared independent. It would enable the government to denationalize more blacks and remove their claim to political rights in what it regards as "white" South Africa.

The government's ultimate aim is to make all of South Africa's 21 million blacks statutory foreigners and turn the 4.5 million whites into a de jure majority.

Buthelezi accused Swaziland of helping South Africa toward this goal, against OAU policy that member states should fight its apartheid (segregationist) objectives in every way possible.