The South African government's detention last week of the president of the country's biggest labor organization marks the latest move in a wave of unrest that began in the segregated black townships 2 1/2 months ago and that authorities say has now reached a more serious level than the Soweto disturbances of 1976.

Chris Dlamini, president of the 150,000-member Federation of South African Trade Unions, was detained Friday along with five others, two of them also labor activists, under a security law that provides for indefinite detention in solitary confinement without charges. All six had backed a two-day general strike last week by black workers that brought much of the country's industrial heartland to a standstill. The strike was to protest the government's heavy-handed response to earlier black protests.

Now leaders of the predominantly black labor movement in this white-minority-ruled country are warning that the government's latest retaliatory action may trigger a black-white industrial-relations confrontation.

"This action will only serve to further inflame and polarize the situation," said Joe Foster, general secretary of Dlamini's labor federation, voicing the concern of many black labor leaders.

This would be a setback for the administration of President Pieter W. Botha, who says he is trying to reform the segregationist system called apartheid. Botha's government points to the emergence over the past five years of a black union movement, prohibited by previous administrations, as the most tangible evidence of its intentions.

The administration, however, seems determined to hold to its policy of toughness, despite evidence that it is escalating unrest, which Minister of Law and Order Louis Le Grange said last week had exceeded the Soweto disturbances of 1976.

During the Soweto riots, which grew out of black student protests against the compulsory use of the Afrikaans language in primary and secondary schools, more than 600 people were killed in nearly four months of violence.

While the death toll this time is lower, the unrest is broader based. Police action during the recent disturbances has brought the number of people detained without charges this year to more than 1,000, the highest in many years, according to an organization called the Detainees' Parents' Support Committee.

The committee, which keeps track of all administrative action taken under the stringent security laws, said 130 people had been killed in the current wave of township unrest that began in August. It also said more than 2,000 people had been arrested and charged under security laws.

In an explanation of the government's position a few days ago, Home Affairs Minister Frederick W. de Klerk said the government would not tolerate "destabilizing activity" in any sphere, including labor relations. He is also a leader of the ruling National Party in Transvaal Province, where much of the unrest has occurred.

The principle of South Africa's new labor legislation, de Klerk said, was that the unions should keep out of politics.

Indicating that the government felt it had to tighten its security grip at a time of reform, de Klerk added: "Political rights will be achieved by everyone through constitutional development . . . . South Africa cannot afford to allow its labor and economic spheres to become a battlefield."

The troubles this fall have grown, in part, out of sharply differing black and white perceptions of political change under Botha's program.

Last August, the government introduced the centerpiece of these changes, a new national constitution that gives a subordinate form of parliamentary representation to the minorities of mixed-race and Asian origin while excluding African blacks, who comprise 73 percent of the population, from any role in the government.

While a few leaders in the mixed-race and Asian communities welcomed the new system, many others called for a boycott of elections for the reconstituted Parliament. This started the confrontation, which has since escalated in a vicious cycle of protests followed by tough official countermeasures.

The boycott succeeded to the extent that 70 percent of the mixed-race people and 80 percent of the Asians chose not to vote. Arguing that the low turnout was the work of political "intimidators," police detained dozens of the boycott organizers, mostly leaders of an alliance of black community organizations and labor unions called the United Democratic Front, which had been formed to campaign against the constitution.

When blacks in segregated townships demonstrated against these detentions, and against new higher rents for their state-owned houses, police retaliated with tear gas and rubber bullets.

As the unrest grew, police action intensified. On Oct. 23, a combined force of 7,000 police and soldiers encircled three townships south of Johannesburg -- Sebokeng, Sharpeville and Boipatong -- and searched the homes of 225,000 people. A week later there was another massive raid. The authorities were looking for "revolutionary elements," Le Grange later explained. In all, more than 350 people were arrested, but they were charged with minor violations unrelated to the security laws.

Last week's two-day strike was called to protest these raids. Significantly, a number of black labor unions, which until then had steered clear of political involvement, heeded the strike call. Among them was Dlamini's federation, which is generally acknowledged to be the most important grouping in the emergent labor movement.

The protest strike was the biggest in the country's history and brought a heavily industrialized region around Johannesburg to a near standstill for 48 hours.

Again, this has elicited a tough response. On Tuesday the giant state-owned oil-from-coal company, Sasol, fired 6,000 black workers -- 90 percent of its labor force -- for joining the strike.