Black nationalist politics is reemerging in South Africa after almost being eliminated by the outlawing of the two main movements in 1960 and 1977.

One of these movements revived in June, with creation of a National Forum out of the fragments of the "black consciousness" movement that was led by Steve Biko before he died in the custody of the security police more than five years ago.

A bigger movement called the United Democratic Front was launched in Cape Town today by more than 100 civic, labor and sports groups that take their ideological cue from nonracist principles of a "freedom charter" drafted at a "Congress of the People" in 1955. The driving force behind that rally was the African National Congress, which was outlawed in 1960 and since then has formed a revolutionary wing to try to overthrow white-minority rule by force.

Members of the National Forum ended their two-day conference in a black township north of Pretoria by signing a "manifesto of the Azanian people." Azania is the name the black consciousness movement would give to South Africa after coming to power.

The manifesto identifies "racial capitalism" as the cause of oppression of blacks in this country and calls for black assertiveness to establish an "antiracist socialist republic."

The 1955 charter also calls for the establishment of a socialist republic, but affirms that South Africa belongs to "all who live in it, black and white."

Today's launch of the Democratic Front also took the form of a "people's rally," attended by about 10,000 persons in Mitchell's Plain, a segregated township for Colored (mixed race) people outside Cape Town.

A principal reason for rebirth of black nationalist politics is the government's proposed constitutional change to give token parliamentary representation to the Colored and Indian minorities but to continue excluding the African majority.

While the government's supporters indicate that they regard the new constitution as an important concession that they expect to begin a process of racial reconciliation, many blacks appear to be uniting in angry rejection of it.

Hennie Kotzer, a political scientist at Johannesburg's Rand Afrikaans University, disclosed Thursday that the progovernment university had done research showing the reform to be even less popular among blacks than the idea of continued whites-only rule.

More than 80 percent of surveyed blacks expressing an opinion opposed a parliament with Coloreds and Indians, with 67 percent opposed to the present whites-only Parliament, Kotzer told a congress of political scientists.

The idea of forming a united front of groups opposed to the constitution began to take shape in January, soon after the government unveiled its reform plan. The main Colored party, the Labor Party, had dismayed many of its supporters by agreeing to participate in the plan.

At a meeting in Johannesburg city hall, Alan Boesak, a young Colored theologian who is president of the World Alliance of Reformed Churches and something of a national hero among young blacks, called for united opposition to the constitution.

"There is no reason why churches, civic associations, trade union, student organizations and sports bodies should not pool their resources and inform the people of the fraud that is about to be perpetrated in their name," Boesak told the meeting.

That is what has happened. Working committees formed and about 100 organizations agreed two weeks ago to form the United Democratic Front. Many wanted Boesak to be the leader, but he declined. A collective leadership includes Albertina Sisulu, wife of Walter Sisulu, the imprisoned deputy leader of the African National Congress, and Oscar Mpetha, an elderly black labor unionist.

Another factor in the political revival was the government's having allowed more than 50 banning orders on black activists to lapse in June. It has failed to ban some others recently released from prison, in contrast with past practice.

The Reagan administration has quietly claimed credit for this apparent phasing out of the system of restriction by banning, which prevents people from participating in politics, attending gatherings or being quoted in the media.

Some South African sources have suggested that it was done to help the Reagan administration justify the continuation of its policy of "constructive engagement." It has been criticized as having failed to achieve relaxation of the segregationist system here known as apartheid.

There are now only 11 persons under banning orders, the lowest number since the system was instituted 25 years ago.

The effect of the relaxation has been to put a number of black political figures back into action. However, six weeks after lifting of the banning order on Sisulu, she was arrested on a charge under the wide-ranging security laws and is in custody awaiting trial in October.

Mpetha too, is facing a three-year prison sentence for a securitty offense and is free pending an appeal.

Saths Cooper and Muntu Myeza are two of Biko's former co-workers who were released from prison recently and not banned. They were elected officers of the Azanian People's Organization, formed in 1978 to fill the gap left by the banning of 19 units that made up Biko's movement.

The new leaders reinvigorated the group, which took the lead in organizing the June conference that led to the formation of the National Forum.

Ideological differences between the two new groupings remain a stumbling block to overall black unity. The issue that has kept black nationalists divided for more than three decades is what relationship they should have with sympathetic whites.

The Congress movement argues that class conflict, regardless of race, is the main dynamic of oppression in South Africa. It has a long history of working with what it calls "democratic" whites, a number of whom have been Marxists.

The opposing view, now reincarnated in the National Forum, is that the roots of oppression are fundamentally racial and that blacks must therefore provide the leadership in the struggle for liberation. Whites, however sympathetic, must be kept at a distance until the struggle has been won.

As a corollary, black consciousness tends toward anticommunism. It advocates what it calls African socialism and nonalignment in the East-West conflict.

Animosity between these two main strands of black nationalism has been fierce and mutually destructive over the years. The reemergent movements appear to be trying to play down the differences, although they are still evident.