It has been, by recent South African standards, a fairly normal week of civil unrest -- 26 black persons have died in six days, slightly below this year's daily average.

What makes the deaths stand out is that all but one of the victims were killed by other blacks, according to the state Bureau for Information. Several of the killings were by the "necklace" method, the grisly execution ritual in which a victim's hands and feet are bound and a gasoline-filled tire placed around his neck and set afire. Two others reportedly were saved from a similar fate by police.

For both the government and its opponents, the question of what is officially called "black-on-black" violence has become a crucial propaganda issue, and the fiery necklace its most potent and troubling symbol.

The deaths fuel Pretoria's assertion that South Africa's unrest is no longer a conflict between a white-minority government and a disenfranchised black majority, but a war among blacks themselves.

Each black-on-black death is cited as evidence that blacks are not ready to govern each other, let alone whites, and furthers Pretoria's claim that it is struggling to resist not legitimate black aspirations but a faceless, barbaric mob that would trample western values and wreak havoc on whites if it ever came to power.

Thus, the information bureau in statements this week categorized the killings as "a desperate reaction of radicals against the restoration of order and to gain a hold over the peace-loving majority by intimidation."

The bureau is the only authorized source of news about civil unrest and police activities during the five-week-old state of emergency here. This article was written under terms of the government's restrictions on press coverage.

Antiapartheid opponents, in turn, contend the government is concealing the fact that many of the killings are committed by blacks considered allied to Pretoria, many of them self-appointed vigilantes who enjoy the tacit, and at times active, support of local police.

Ultimately, these opponents hold the apartheid system responsible for creating and capitalizing on splits within the black community and for a process of brutalization that has transformed many young blacks into urban warriors who condone, and even celebrate, the necklace ritual.

"I believe necklacing is horrible and I can't agree with it," said Seth Mazibuko, a top official of the Soweto Civic Association, which is affiliated with the United Democratic Front, the country's leading antiapartheid coalition.

"I understand why people do it. The apartheid system puts such a terrible anger into the people it hurts. But it actually stigmatizes our struggle."

But there is an uncomfortable ambivalence among many blacks about the war inside their segregated townships. While disliking the necklace, many believe it has served to erode and frighten off the vast network of police informers that has been a major factor for the state in the past in undermining the organization of an effective opposition.

"We want to make the death of a collaborator so grotesque that people will never think of it," said Tim Ngubane, an official of the outlawed African National Congress, in a speech in California last October.

Winnie Mandela, wife of imprisoned ANC leader Nelson Mandela, reportedly told a gathering of black mourners at a funeral in April, "With our boxes of matches and our necklaces, we shall liberate this country."

One measure of the intensity of the propaganda war being waged here is that both alleged statements were repeated by the government this week in its effort to characterize black-on-black unrest as the last gasp of desperate militants.

As the daily death rate has doubled during the past year, the percentage of deaths attributable to black-on-black violence also has risen. Between September 1984 and January 1986, according to South African police, nearly 60 percent of the deaths were blacks killed by security forces. But for the first six months of this year -- according to the police and independent researchers -- the figure dropped to about 33 percent.

Since the emergency took effect on June 12, the information bureau contends that more than 75 percent of the 157 deaths have been blacks killed by other blacks. Because of the emergency restrictions, it is not possible to verify those figures independently.

According to the government, the necklace has gained in popularity as the death toll has mounted. In his speech announcing the new emergency to Parliament last month, President Pieter W. Botha said that between March and June, 284 blacks were killed by radicals, 172 of them by the necklace.

Many opponents argue that police distort the numbers by underreporting black deaths, an allegation that police officials deny. Others contend that the figures reflect the shift in state strategy away from white law enforcement to rule by black vigilantes.

The state's black-on-black figures, they note, do not offer any breakdown of who killed whom, and thus lend little support to the contention that radicals are responsible for most of the deaths.

In places like the Crossroads squatter camp near Cape Town, where right-wing vigilantes known as "fathers" battled leftist "comrades" in a power struggle that left more than 50 dead in May and June, police appeared to give strong support to the conservatives. Similar conflicts have erupted in black "homelands" such as KwaNdebele and Bophuthatswana, where President Lucas Mangope recently exhorted his followers to "protect what you have" from young radicals.

The black-on-black issue lies at the heart of the new state of emergency, which the government says it imposed to restore order and end the rampant intimidation that terrorizes the silent majority in many of South Africa's black townships.

But churchmen like Bishop Desmond Tutu and UDF leaders say the police crackdown and the arrests of at least 4,000 activists, many of them with UDF affiliations, have had the opposite effect by taking off the streets those who can impose a measure of order on younger militants.

Soweto UDF leader Mazibuko, interviewed in downtown Johannesburg where he has been in hiding from the security police for several weeks, described how youths were enforcing a new rent boycott by going from house to house and threatening elderly residents with burnings if they pay.

"We've got to stop this one or else there could be chaos," he said. "They wouldn't do this if members of the Soweto Civic Association were still around. But we're all in hiding or in jail."

Mazibuko and other UDF leaders also are troubled by increasing conflict between their supporters and the followers of Zulu Chief Mangosuthu Gatsha Buthelezi. More than 60 persons died in Durban last August in clashes between Buthelezi's Inkatha movement and UDF members, and a low-scale war has simmered since.

But few of the government's opponents would argue with Buthelezi's words at a rally three weeks ago.

"If we do not do something about the high toll of deaths of blacks at the hands of blacks," Buthelezi warned, "we are on the verge of a civil war situation which will never be stopped, even if liberation is achieved tomorrow . . . I tell you bluntly today that we will never win the struggle for liberation if we divide ourselves one from the other through violence."