With the extension of its emergency decree to Cape Town and surrounding townships, the South African government has sent a message of implacable defiance to its black opposition, white business critics and the world at large.

The action appears to limit severely whatever hope existed that the government would show that it was listening to its western creditors and allies by easing or even scrapping the decree and offering new concessions that might persuade black moderates to come to the bargaining table for talks about the country's political future.

Instead, President Pieter W. Botha, clearly fed up with international criticism and unwilling to be seen bowing to outside pressure, appears to have chosen to tough it out. Having failed to crush two months of political unrest in western Cape Province through the use of official force and sweeping arrests, he has fallen back on a traditional South African approach -- more force and yet more arrests.

The government announced that all journalists are banned from entering Soweto, the country's largest black urban center, until further notice. No reason was given, but officials have accused foreign reporters of distorting the picture in coverage of unrest in black townships and contributing to South Africa's deteriorating image overseas.

The new emergency proclamation extends to Cape Town and seven other western Cape areas the broad emergency police powers already in effect in 30 other districts, including Johannesburg and Port Elizabeth.

One immediate victim of the extension is likely to be the government's campaign to settle its foreign debt crisis. At an opening round of talks in London Wednesday, foreign bankers made clear to South African officials that the debt problem, which has caused the government to freeze payments until at least January, cannot be solved until the country's political crisis eases. Instead, the extension of the emergency sends a strong signal overseas that the crisis is deepening.

The move undoubtedly will play better at home with white voters in the five districts where parliamentary by-elections are being held next Wednesday, and it may help the government hold onto all five seats despite a strong challenge from the right.

Recent polls suggest that whites are increasingly uneasy about the future and inclined to fall in behind tough measures such as the emergency.

But many analysts believe that the reason for the get-tough approach lies more in Botha's personality and world view than in any calculation of political advantage.

As he has made clear in recent speeches, Botha believes that he has gone as far as a white Afrikaner leader can go in offering concessions such as the restoration of South African citizenship to blacks in "homelands" and modifications of the country's notorious pass laws controlling black mobility. The pillars of apartheid -- strictly segregated neighborhoods and schools, race classification and white control of Parliament -- will remain intact at least until Botha, 69, steps down.

He sees black violence, which during the past 14 months has claimed more than 800 lives, as a product not of genuine discontent but of Marxist radicals bent upon destroying white civilization. He also seems bewildered and hurt that others in the business community and in the West -- especially the formerly friendly Reagan administration -- neither share his view nor applaud his steadfastness.

Having forfeited their support, he is not inclined to beg for its return. Instead, he has chosen to follow his instincts -- and the pattern laid down by Afrikaner leaders in similar moments of crisis in 1960 and 1976 -- by cracking down.

"We are not accepting dictates from any country . . . and we are not going to allow interference in the internal affairs of South Africa," an emphatic Botha told a cheering audience at a political rally Thursday night in the town of Springs, one of the districts having elections next week. "We live in stormy times, but I've been in public life a long time, and I'm not scared of small storms."

When western diplomats pleaded for clemency for convicted black nationalist Benjamin Moloise, sentenced to hang for killing a black policeman, Botha ordered his execution.

When businessmen and opposition white politicians traveled to Lusaka, Zambia, to meet with the exiled black leaders of the outlawed African National Congress, Botha questioned their patriotism and mocked their efforts at conciliation. The government has indicated that it will not allow such meetings to take place again.

When a Cape Town member of Parliament, senior opposition member Colin Eglin, pleaded with Botha to talk to community leaders in an effort to quell unrest in the city, Botha responded by accusing Eglin's Progressive Federal Party of helping stimulate the violence. Instead of talks, the government introduced a new water cannon, automatic-firing armored vehicles and a special riot-control helicopter to the Cape Town area last week.

Eglin said last night that the extension of the emergency was "a searing indictment of Mr. Botha's government and an admission that after weeks of bungling and bullying, government policy has finally collapsed."

Cape Town is South Africa's jewel and the cradle of this troubled, white-ruled society. It is the country's most beautiful city and its most cosmopolitan, home of most of its mixed-race, or Colored, population and a place where racial attitudes supposedly are more relaxed than in the interior.

But for two months, the youths of Cape Town's tidy, neatly trimmed Colored neighborhoods east of the city have been at war with the police. When it started in late August, police mostly used tear gas, rubber bullets and plastic whips, while the youths wielded rocks and occasional gasoline bombs.

Now, police regularly use live ammunition, and rioters in some cases have begun shooting back. Two policemen have suffered gunshot wounds, and last Wednesday, two hand grenades were launched at police patrols. At least 65 persons have been killed.

The irony is that Botha's government has sought to forge an alliance with South Africa's Coloreds, giving them their own house in Parliament. The assumption is that if the government cannot sell this community on its "reform" program, it cannot sell anyone, especially not the black majority.

The revolt here is one rooted in rising middle-class expectations rather than in poverty, of people who, having been granted limited rights, are even more eager for full ones. Thus, many critics see the extension of the emergency to Cape Town as a tacit admission by the government that its policy of limited reforms has failed. If so, it is also an indication that the policy vacuum left by that failure is to be filled increasingly by the use of police power.

Frankel also reported:

Two more blacks were killed by police during unrest in the Cape last night, and police reported incidents of stoning, gasoline bomb attacks and the erecting of burning barricades in at least a half dozen Cape area townships.

In Soweto, 19 white women, many of them mothers of national servicemen, and two blacks were arrested for staging a demonstration outside a police station calling for the withdrawal of troops from black townships. Most of the women were released on bail tonight.