The knock at the door came at 3 a.m. Six policemen stood in the southern winter chill waiting for the trade union leader. "Go and say goodbye to your wife," one of them told him.

He said a few words of comfort to her, kissed his sleeping children, grabbed a coat and headed out the door. The scene at the matchbox house in a black township of Port Elizabeth was repeated somewhere between 1,000 and 2,000 times Thursday night as South Africa's new, nationwide state of emergency took effect, and hundreds of antiapartheid activists were rounded up.

It was the largest crackdown on political opposition ever carried out in this white-ruled country.

Its immediate purpose was to undercut plans by the activists to commemorate Monday as the 10th anniversary of the Soweto uprising with a nationwide strike and protests that police feared they would not be able to control.

But underlying the move was something more -- a sense among the country's white leaders that the time had come to ignore, even defy, international opinion and use the full range of state power to quell South Africa's two-year-old civil unrest.

Several factors pushed President Pieter W. Botha into acting now, according to analysts here. One was the fear of a growing right-wing backlash among his bedrock white power base. Another was the concern that the government's efforts to build a constituency of black supporters among those it considers "moderates" were being undermined by the threat of retaliation by radicals prepared to burn anyone considered a "sell-out."

Equally important, some analysts said, was Botha's visceral anger at his internal critics and at the continuing threat of international sanctions. "There is a feeling among the top Cabinet ministers that they are tired of being threatened and that it is time to show the world who is calling the shots," said Afrikaner political scientist Hermann Giliomee.

As Botha made clear in announcing the tough new measures, he is determined that his government not join history's scrap heap of discarded rightist regimes. In his view, those governments -- and he named South Vietnam, Nicaragua and Iran, among others -- took too much heed of bad advice from western states urging restraint against a revolutionary foe.

He approvingly cited Henry Kissinger, who wrote in his memoirs: "When the crying need is for an assertion of authority, our advice usually dilutes it. And hard-pressed governments beset by an implacable domestic enemy are often reduced to paralysis by advice which they know is dangerous if not disastrous but which they dare not reject."

Botha long has contended that his government has used only a fraction of its firepower in putting down the long-simmering black revolt. He has told interviewers that he believes both black opponents in the outlawed African National Congress and western critics have mistaken his restraint for weakness and have drawn the conclusion that the state will buckle under more pressure. The emergency clearly seems designed to put to rest this belief.

From a diplomatic viewpoint, the measures could not come at a worse time. They undoubtedly will fuel the momentum begun last week by a Commonwealth report recommending collective action against South Africa, and the vote for new U.S. sanctions by the House Foreign Affairs Committee in Washington.

In a speech before Parliament and a televised address later Thursday night, Botha appeared almost to welcome the prospect of new sanctions. "South Africans will not allow themselves to be humiliated in order to prevent sanctions," he said. "If we have to be dependent on our Creator and our own ability alone, then I say let it be."

In declaring his defiance, Botha offered cold comfort to western leaders such as Ronald Reagan and Margaret Thatcher who have sought to forestall sanctions.

He told them he would not accept "any holier-than-thou pontification from whatever quarter," adding, "Your reward is not to be found in any right to dictate the contents of South Africa's domestic policies . . . Your reward will be what it ordinarily should be -- a reliable trading partner and a reliable supplier."

Those statements reflect the views of a small coterie within the Cabinet that is now almost singlehandedly forging government policy, according to Giliomee. The group includes Botha, Law and Order Minister Louis le Grange, Commissioner of Police Johan Coetzee and Minister of Constitutional Development Chris Heunis. It pointedly does not include Foreign Minister Roelof F. (Pik) Botha and other Cabinet members identified with a more conciliatory approach.

It was Pik Botha and the moderates who had pushed for a breakthrough from the Commonwealth's Eminent Persons Group last month, western diplomatic sources said. The seven elder statesmen, appointed to seek to play a mediating role, had proposed to Pretoria that the African National Congress be legalized and its imprisoned leader, Nelson Mandela, released, sources close to the group said. In turn, the congress would suspend its guerrilla operations and then the two sides would begin negotiations.

The South African government appeared initially to respond positively to the proposal, as did Mandela, who sounded an especially moderate and reconciliatory note during the group's meetings with him in prison.

The group returned to Cape Town in mid-May. Instead of the meeting they had expected to hold with President Botha, they were advised to attend a speech in which Botha slammed the "unsolicited interference" of "meddling groups visiting the country."

Four days later, after the group returned to South Africa from meeting with congress leaders in Zambia, Pretoria raided alleged ANC sites in that country, Botswana and Zimbabwe. The seven returned to London. Many here fear that the failure of their peace mission will prove a grim marker on the road to increased bloodshed.

The new crackdown poses a challenge to the government's opponents, who must again devise strategies and plans while an important segment of their leadership is behind bars. Last year's emergency only seemed to feed popular disaffection in the segregated black townships, drawing the lines more sharply between government opponents and supporters and shrinking the limited space in between for those who oppose the apartheid system yet do not favor revolutionary violence.

The government clings to the notion that those moderates can be enticed to its bargaining table once a small but vociferous minority of radicals can be brought to heel. It is a view that most critics, including those in the Eminent Persons Group, believe grossly underestimates the extent of black anger.

But the activists also must find ways of coping with the ever deepening divisions within black ranks. Many were quick to blame the police for supporting right-wing black vigilantes in their campaign of terror in recent weeks to rid the Crossroads squatter camp outside Cape Town of young, leftist militants. But some also conceded that the police merely were capitalizing on deep and genuine resentment among older, more conservative blacks at the arrogance and intolerance shown by the radicals who had sought to enforce a new revolutionary order on South Africa's shantytowns.

Besides the activists, the emergency's other target is the press. By ordering the expulsion of a CBS cameraman and by warning foreign journalists that their reports are subject to the emergency regulations, Pretoria appeared especially keen to put the foreign media on notice that they were in jeopardy.

For many activists, the roundups had a chilling sense of familiarity. Many of those picked up had been arrested in last year's crackdown as well. Several arrested in the Port Elizabeth area are plaintiffs in a civil suit brought by Dr. Wendy Orr, a former prison doctor, against the police alleging that they were severely assaulted and tortured during their detentions. Now they have been detained again by the very police whom they have accused.

One of those most prominently named as a torture victim in that lawsuit was the trade union leader mentioned in the first paragraphs of this article. The Washington Post has not named this man, a top official of an automotive workers' union, because the new emergency regulations prohibit the unauthorized publication of the names of detainees. Penalities for violating the order include up to 10 years in prison and an $8,000 fine. So far the government has refused to release either the names or number of those arrested.