No sooner had the Colored (mixed-race) political leader begun explaining his party's decision to participate in the South African government's new reform constitution last week than fists, chairs, bricks and stones began to fly.

Five people lay badly injured, some with knife wounds, by the time police arrived to protect Labor Party leader Allan Hendrickse and his deputy, David Curry, at a meeting in the pretty country town of Stellenbosch, near here.

It was the second time in a week a Labor Party meeting had ended violently in a country town, where Coloreds have always appeared subservient and almost as conservative as the white "masters" for whom they work in this segregated society.

Most of the militants are found in the cities, so it was not surprising that after their experiences in the countryside Hendrickse and Curry cancelled their scheduled meeting in Cape Town Thursday night.

"We shall return when feelings have simmered down," said Hendrickse in his announcement.

But it seems doubtful whether feelings will simmer down, or whether the Labor Party leaders will ever feel safe in addressing public meetings in this southwestern corner of Cape Province where 80 percent of South Africa's 2.5 million mixed-race people live.

There appears to be a groundswell of anger against the party for agreeing to participate in the new constitution, which offers token parliamentary representation to the Colored and Asian minorities but not to the 21-million black majority.

The Labor leaders say they want to go into the proposed new racially segregated tricameral parliament to use it as a platform to fight for more rights for nonwhites, including blacks, but their critics accuse them of betraying the black cause and becoming "junior partners in white supremacy."

The dispute is generating intense passions. Allan Boesak, a young Colored theologian who is emerging as the central figure among those known as anticollaborationists, thinks it could develop into the worst conflict South Africa has known in years, culminating in a government crackdown and a new crop of oppressive security laws.

"This time it won't be a conflict along straight racial lines," Boesak said in an interview here. "It will cut across them. But it will be no less violent or tragic in its consequences because of that."

This prospect makes for a bitter irony. Opinion polls indicate most whites think the new constitution is a genuine reformist step that will lead to a better deal for all nonwhites. They are giving Prime Minister Pieter W. Botha credit for having the courage to split his ruling National Party to introduce it.

The U.S. State Department has cautiously welcomed it as a step in the right direction, and the Labor Party's decision to participate gave it the impression of legitimacy. Labor has long been the main party among the Coloreds.

Yet if Boesak is right, then what most whites see as a step toward racial reconciliation is being seen by most blacks as a step toward deeper polarization and conflict.

Chief Gatsha Buthelezi, most important of the moderate black leaders, has said its implementation will amount to a ganging up of the white, Colored and Indian minorities against the black majority, with the whites still in overall control.

Buthelezi has denounced the Labor Party for joining in and seems certain to end his partnership with them in a multiracial grouping called the Black Alliance.

Boesak believes the only way to lessen the coming conflict is to organize a boycott, so that when the government goes ahead with its constitution, probably some time next year, it will do so with a minimum of support from the Colored and Asian communities.

To this end, he is helping in the formation of a new alliance of anticollaborationists, called the United Democratic Front.

The front was launched at a recent meeting in Johannesburg. A number of black labor unions have also joined--the first overt political move by South Africa's potentially powerful black union movement, which began emerging when racially restrictive labor laws were relaxed four years ago.

It seems inevitable that this development is going to draw Boesak into the role of a major new political leader of blacks, although he insists he does not want this.

Until now he has confined himself to church affairs. Last year he campaigned in the World Alliance of Reformed Churches for the suspension of South Africa's influential Dutch Reformed Church, to which most government supporters belong, because of its theological endorsement of racial segregation. After succeeding, Boesak was elected president of the world alliance.

His success made him a hero among the younger, more militant Coloreds. At 36, and as student chaplain at the Colored university here, he is someone with whom they can identify.

Now his role in the anticollaborationist movement is boosting that image. He is an effective orator in the mold of Martin Luther King Jr. and his quotes on the issue are being repeated like slogans. The crowds chanted his name during the uproar at the two country-town meetings.

Boesak says he sees his role in the church, not politics, but he is already being invoked as the spiritual leader of the new movement.

Whether he likes it or not, he looks like being thrust into a high-risk position on the political front line.