When they held 25 whites hostages in a Pretoria bank last month, three armed black men unwittingly made South African Prime Minister Pieter W. Botha appear to be a prophet.

In August, Botha warned his fellow whites that unless they accepted reforms in their strict apartheid society, they would face "revolution" in South Africa. The bank siege, in which two females hostages died with their captors when police stormed the building, seemed to prove his point.

But prophetic powers were not needed to forecast an event like this one, which has sent shock waves rippling through the white community. The pressures have been building for some time.

In a study of 1976 and 1977 political trials, South African lawyer Glenn Moss concluded that since June 1976 "the level and intensity of conflict . . . has grown enormously and is taking on the proportions of a low intensity civil war." Moss noted that since mid-1978 there has been a steadily increasing return of armed guerrilla fighters, many of whom went into exile during the riots of 1976.

Some observers regard the bank siege as a portent of the kind of armed conflict blacks will wage against the government, concentrating their activities in urban areas, unlike guerrilla wars in neighboring states such as Rhodesia that have been mainly in the countryside. Moss compiled a random list of politically violent incidents since 1976, most of them bombings, that show 60 percent were in urban centers.

The Pretoria bank siege is regarded as significant for another reason. Reactions to the hostage-taking illustrate the gaping division in South African society between the powerful, yet threatened whites and the powerless, angry blacks.

For whites, the black captors were simply "terrorists" and whites were angry when others did not see them so.

"When are you going to start calling them 'terrorists'?" a government official angrily asked me after reading an account of the episode in The Washington Post that referred to the three men as "nationlists." The police identified the men as members of South Africa's oldest black nationalist organization. The African National Congress.

The chief of police referred to one of the men as "a swine," and a white spectator outside the bank was heard to say as he looked at the body of one of the men lying in blood on the bank floor, "There lies that black rubbish."

But blacks had different opinions and several thousand of them expressed it when they attended the funeral of one of the dead men, Fannie Mafoko in Soweto Feb. 9. They sang freedom songs, raised their clenched fists in black power salutes and carried placards praising him as a "hero." Among South African newspapers the black daily, The Johannesburg Post, stood alone in referring to the men as "guerrillas."

Privately many blacks expressed their "disappointment" that the gunmen had not "taken more hostages with them."

"They should have killed them all," said one black woman in her 50s. Her comment and others like it voice the reluctantly accepted, but growing tolerance in the black community of violence as the only way to force the white minority government into making meaningful changes in its racially discriminatory system.

"Those of us who are working for justice, equity and reconciliation through peaceful means are being steadily discredited," said Anglican bishop Desmond Tutu. Like other black leaders who deplored the violence of the black siege, he warned that more such incidents would occur unless the government matched the dissidents' boldness with bold reforms.

But no one in the government, not even Botha, has made the connection between the bank siege and his predicted revolution. Officials and decision-makers appear to have not yet sensed the urgency of black frustration and rage.

Two attempts by The Washington Post were unsuccessful to solicit comment from reformist-minded black affairs minister Piet Koornhof -- usually conscientious about his contacts with the press -- on the relationship between the bank siege and the pace of promised reform.

One Cabinet minister not identified told a local paper: "We will not be dictated to. We will continue developing our policy in a reasonable time." Another thought the huge crowd at the Soweto funeral was due to "intimidation" and a third thought the different perceptions about the bank siege were only between a minority of blacks on the one hand and everyone else on the other.

No doubt there are some within the reformist faction of Botha's National Party who agree with the Afrikaans-language newspapers, which usually support the government, that the bank siege showed the need for rapid constitutional change. But those officials are not saying so in public becaus of a vociferous and burgeoning rightist backlash against Botha's announced reform.

Instead, official statements have concentrated on discouraging such acts as a means of political protest, which explains the decision to put a quick end to the hostage situation by storming the bank and setting off the shooting that left five dead.

Police commissioner Michael Geldenhuys defended this strategy by comparing it to American action involving hostages held at the embassy in Iran. He said, "We certainly did better than the Americans have done in their long hostage agony."

Police minister Louis le Grange refused to publish the demands of the three men saying that would be "tactically incorrect" since this is what the men had wanted.

He has also given notice that he will introduce legislation to ban press reporting of political demands and of police tactics in similar incidents in the future. Extensive coverage was given to how the police bugged the bank and observed the gunmen's actions through a periscope during the six-hour ordeal.

Yet the fact remains that despite their widespread security and informer system, the police could not prevent the Pretoria bank siege. And although one of the armed captors was a "wanted man" since he fled the country in 1976, they failed to net him when he came home clandestinely in December to spend three days with his girlfriend in Soweto.