S. African anti-apartheid rally: a watershed in black power?

Buds foretell spring across South Africa. And with them this year comes an apparent blossoming of broad-based black opposition to the government's policy of apartheid.

At the same time, apartheid is being fitted for new clothes by the government of Prime Minister Pieter W. Botha. In the next few weeks South Africa's all-white Parliament is expected to pass a new constitution bringing the Colored (mixed race descent) and Indian population groups into the central government. The black majority would still be excluded.

Blacks are rallying to defeat the plan, which will go before whites in a referendum. Coloreds and Indians may also vote on it, the government says.

Leading the opposition for blacks is the new United Democratic Front, launched at a weekend rally. Its formation is considered a potential watershed in black politics, although the UDF's effectiveness is still uncertain.

The UDF is a modern alliance of some 400 organizations, ranging from the South African Council of Churches to the Council of Unions of South Africa and the Azanian Students Organization. The UDF is principally a black political initiative, although its membership is multiracial.

The new organization's roots go deep, back to the so-called Congress Alliance movement of the 1950s. That movement brought together a broad range of government opponents, from all race groups, in a massive resistance campaign. The UDF is the first real attempt to revive that broad ''nonracial'' strategy of government opposition. The ultimate aim: a ''united and democratic'' South Africa.

Although the UDF eschews violence and operates openly, the government is watching it with an eagle eye. The Congress Alliance movement of which the UDF is so reminiscent included the government's No. 1 enemy: the banned African National Congress (ANC).

The UDF joins a long list of groups opposing the government's new constitution. In the white Parliament itself, main opposition parties - the Conservative Party on the right and the Progressive Federal Party on the left - reject the proposals. But the ruling National Party holds an overwhelming parliamentary majority, so the constitution is expected to pass.

The only strong ally the government has managed to enlist behind the ''power-sharing'' proposals is the Colored Labor Party.

Opposition from the left, overwhelmingly more significant in national terms than right-wing resistance, stems from the belief the new constitution makes no headway on the central issue facing South Africa: how to give the black majority some real power.

''To be sure, the new proposals will make apartheid less blatant in some ways ,'' said Allan Boesak at the UDF's launching. ''But for those of us who are black and who suffer under this system, there is no positive side'' to the so-called ''reforms.''

Dr. Boesak, a Colored theologian and president of the World Alliance of Reformed Churches, first suggested the idea of a UDF earlier this year.

The government's stance is that blacks need not be included in central government because they have been granted political rights in designated ''homelands.'' But most blacks reject the concept of the homelands.

While political analysts consider the UDF important as an exercise in black political unity, its rockiest road now lies ahead. It appears to have considerable grass-roots support and drew a crowd of at least 6,000 to its weekend rally.

By one count, its member organizations represent a force of at least 1 million.

But like any black political initiative, it is severely handicapped by government restrictions. Its opening rally near Cape Town, for instance, was crammed into a community hall and a tent because outdoor political gatherings are illegal in South Africa.

It would also be illegal for the UDF to be proclaimed an official political party since multiracial political parties are banned.

The greatest threat hangs over the UDF's leadership, vulnerable to detention or other security actions if they are not extremely careful in how they conduct their campaign to defeat the constitutional proposals.

All the UDF will say now is that it plans to try to educate people so they ''do not fall for the government propaganda.''

Three veterans of black resistance campaigns were elected national presidents of the UDF: Archie Gumede, former executive member of the ANC; trade unionist Oscar Mpetha; and Albertina Sisulu, wife of imprisoned ANC official Walter Sisulu.

Mrs. Sisulu has been charged with furthering the aims of the ANC and is awaiting trial.