Jacob Mashabane, a brilliant but emotionally troubled 22-year-old university student who does not drive, is arrested in downtown Johannesburg. The charge is attempted car theft.

"Jackie was very political, and very upset by all the killing," a friend recalls later. "He could have been fooling around. Or he could have said something a white policeman didn't like. Jakie couldn't keep quiet about what they were doing to us."

Oct. 4: Mashabane appears in court. Bail is set at $230, a sum his family can raise easily. He does not contact them after the hearing, however, and is returned to jail.

A government "appointed coroner conducts a post-mortem two days later and police declare Mashabane a "suicide".But a week passes before his family is notified, on Oct. 14, that the youth was able to fashion a noose from his shirt and hand himself in a crowded, tightly controlled jail. The family get lawyer, but the delay has reduced the chances that an independent autopsy will uncover vital evidence for the still-pending formal inquest.

No fewer than 10 South African blacks have managed to commit suicide while in detention under the normally watchful eyes of the South African police in the past year.

Some have used trousers or shirts or blankets to strangle themselves in cells. Others have become suddenly clumsy and fallen six flight down a stairway. One man broke his neck by falling over a chair while four security policemen were discussing politics with him.

The policemen stuck to that story even after an independent pathologist testified that the dead man's neck injuries more closely resembled the results of a karate blow than a fall. They are still members of the police force.

That police force, armed with the Western world's most inventive and widest range of security laws, is a feared and relentless protector of white power and privilege in South Africa.

Press releases from the white government's New York public relations firm and speeches by its envoy at the United Nations attempt to project to outsiders a new, softer image of a South Africa ready for major peaceful change.

The view from the nation's jail cells is different. The white police force continues to crush dissidents and jail black people who demand a swift and complete end to aprtheid.

This brand of law and order has been a massive block to peaceful change here. The number of what amount to black political prisoners who die in detention and the consistent accounts of physical abuse and even torture told by many of those released have spread a paralyzing fear of political activity through the black community.

A well-paid and widespread network of black informers has intensified these fears. White police spies also penetrate white university student associations, moderate church groups and, according to former Cabinet minister Albert Hertzog, far right-wing dissident groups as well.

The police do not have to jail all dissidents. Under South Africa's legal system, apartheid foes can be confined to their homes, effectively barred from working or kept from publishing, teaching or saying anything to a newspaper reporter, without being told why or being given a chance in court ot contest such restrictions.

Although its security apparatus rivals that of the Soviet Union in single-minded and ruthless suppression of dissidence, in some important aspects South Africa does not fit the populat image of a police state, at least as far as the 4.3 million whites are concerned.

An opposition exists in the all-white Parliament, white-owned newspapers attack the government and a white judiciary functions to the extent that the vague, all-encompassing laws permit. Moreover, the white Afrikaners who control the government have a passion for justifying their actions by passing lawa to cover practices that other states perform only in secret.

Despite these external trappings of a Western parliamentary democracy, the National party government has for nearly two decades used unfettered police power as a substitute for responding to the fundamental political demands of urbanized Africans. The racial upheavals of the past year have clearly moved South Africa closer to becoming a total police state.

Police shootings and mass arrests broke the peaceful anti-apartheid campaign that came to a head in the township of Sharpeville in 1960, and police have responded with the same kind of display of massive force since June, when students began protest against apartheid.

Inspection of official lists of casualties and interviews with South African students, clergymen, reporters and other present during the violent confrontations that have left at least 375 blacks and no police dead, as well as with police officials and lawyers, establish these conclusions:

Maximum force, including shooting to kill unarmed demonstrators, was used frequently not to protect life or property but for the political purpose of ending protest against apartheid. This appears to have been sanctioned by the higest levels of the government, which said the portests had to be stopped "at all costs."

Many of the estimated 540 persons arrested under the three principal security laws last yeat also appear to have been arrested as part of an attempt to breal a political movement, rather than to punish lawbreaking. Justice and Police Minister James T. Kruger acknowledged in an interview that he used the security laws to detain dissidents when he did not have proof to justify a court case.

Deaths of political prisoners in jail cells have risen sharply following the black power upheavals here.

Since 1963, when the Institute of Race Relations began keeping records on this grim subject, 30 prisoners being held by security police are known to have died in detention. One of that group was listed by the government in 1969 simply as "an unnamed person who died in an undisclosed place on an undisclosed date in 1968."

That case and the death in detention of Imam Haron, a Cape Town Moslem religious leader later in 1969, stirred public concern and publicity about police tactics here. Between 1970 and the beginning og 1976, only two persons are known to have died while in the hands of the security police.

But as the black power political movement began to spread in 1976, political activists taken prisoner again fell back into the unusual and at times fatal habits of the previous decade. Lawyers and medical experts who tried to help their families found themselves behind bars.

Joseph Mdululi was the man who stumbled fatally over a chair. An independent autopsy on the 50-year-old activist was arranged by his lawyer, Mungifi Mxenge, who was promptly arrested and held for three months without charge. Publicity around the case the findings of the independent post-mortem, which strongly suggested that Mdululi had been beaten to death, led to the four policemen being taken to court to be cleared of murder charges.

Mapetla Mohapi, a Black Consciousness advocate, died in August, according to police, by hanging himself. Two black physicians who examined his body were arrested before lawyers could have them testify at a formal inquest, now set for later this month.

Others who have died this way in recent months include a 16-year-old schoolboy who was arrested after participating in a protest against Secretary of State Henry A. Kissinger's visit to South Africa; a 17-year-old youth whose body the police refused to return to his parents; a 35-year-old businessman who died 48 hours after turning himself into the police in response to a message they left for him at his house, an da 32-year-old social worker who was dead two hours after his arrest.

The deceased had been detained under the Internal Security Act, the General Law Amendment Act or the Terrorism Act. The first two permit police to hold suspects without charges incommunicado, for seven to 14 days, during which they can be grilled behind prison walls. Serious suspects are then charged under the Terrorism Act, under which their arrest can ne kept secret, access to lawyers denied and their detention prolonged indefinitely.

While suspects charged with robbers, rape or even murder are rountinely granted access to lawyers and the chance to argue for bail, these rights are routinely denied to political prisoners. Only government employed magistrates visit them during the interrogation phase, when the accusations of torture are most frequent.

Justicce Minister Kruger refused to give the number of persons whose preventive detention he had ordered during the upheaval, but said many of them "held leading positions in various organizations that were working behind the scenes during this thing. When you can't get enough evidence on them, you remove them out of the area of unrest for the period of the unrest. That is all we have done."

Kruger, who has proposed that it be made a crime here to publish the names of persons arrested under security laws, added that his decisiona are reviewed by a committee on which former magistrate and two other sit. The committee, which meets secretly and cannot be challeged in court, has never disagreed with any of Kruger's decisions.

The polital cases are separate from the 4,250 to 5,000 arson, public disorder, sabotage and other cases that have arisen from seven months of demonstrations and confrontations with the police.

Of 1,400 of these cases decided before the end of the year, slightly more than half resulted in defendants being beaten as part or all of their sentence. At least 52 defendants under 18 years old - including an eight-year-old boy - received up to 10 cuts with a cane, usually the same day of sentencing and often without having been represented by lawyers.

Lawyers who handled some of the riot cases report that they saw in cells other students still awaiting trial with black eyes, cuts and lacerations and other sign of beatings. "I wasn't their lawyer, so there was nothing I could do, and without a lawyer there is nothing they can do," said one attorney.

The aim of the police campaign appears to go far beyond the mere restoration of public calm.

"The police have to put the fear that paralyzed them since Sharpeville back into the blacks," a white sociologist said.

"They think they can shoot, arrest and beat the black back into submission, and on the past record, you have to conclude that they are right. The government has not used one-half of 1 per cent of the force available to it."

One chilling sign of a return to post-Sharpeville paralysis is the distrust spreading through the ranks of the student movement that lauched the first waves of protest spontaneously and then did a good job in protecting its leadership from the informer network that had crippled their parent's political movements.

Hundreds of students arrested in dragnet sweeps in November heve been released in recent weeks after lengthy interrogation, but they are being shunned by student who escaped arrest.

"There is only one way they could get out, and that is to promise to spy for the police," said one activist. "Many of them have been tortured, and you can't blame them. But you can't trust them. either."

There have been some significant shifts in police tactics here over the past decade and especilly since 1974, when black governments came to power in neighboring Angola and Mozambique and black power became much more of a direct threat.

Censorship has become more systematic and mechanical and publishers more reluctant to risk publishing controversial books likely to be banned. A personable, well-known retired judge formally oversaw a censorship system based mostly on citizen complaints. But sow the censorship board has been handed over to an invisible bureaucraft who evidently receives guidance from the Bureau of State Security.

The bureau, known here as BOSS, is a domestic intlligence agency answerable only to Prime Minister John Vorster, who controls it directly. Created in 1969, the bureau has helped add a distinctly Machiavellian and manupulative gloss to the once straight-forward white rule of South Africa.

The urban uprisings have also triggered a new spate of "bannings," the dinstictive South African practice under which the Justice Ministry can turn opponents into nonpersons.

Nothing written or said by the 134 persons currently serving five-year banning orders may be published or repeated at public gatherings. The banned are forbidden to communicate with one another. Many, like Robert Sobukwe, a leader of the 1950s passive resistance campaigns, are confined to home for 12 hours a day, and may not be visited during that time.

Sobukwe, after serving three years in the Robben Island maximum-security prison for political activities, was imprisoned for six more years after his sentence expired in 1963, when Parliament passed a law each year that had only one effect: to keep Sobukwe in jail, even though he had not been charged with any offense.

This unusual, highly visible procedure produced some complaints from the Englis-speaking white community. But, like the police killings of students and the widespread arrests of the past seven months, it was not criticized by any prominent members of the dominat nt Afrikaner white community, which appears to support fully whatever the polive have done.

"The Afrikaners are people of the book. They have to have a law or a quotation from the Bible to justify what they do," said and Afrikaner educator. "Thhat is why the Afrikaner is different. The Afrikaner has substituted his conscience for the rule of law, in good faith, but with consequences outsiders do not understand."

Since Sharpeville, black here feel that they have a clear understanding of those consequences.

"You do what they want you to do or you go to jail," said black journalist when asked if Johannesburg had a 10 p.m. curfew on blacks similar to that in smaller towns. "They have no time to reason with you. Either you are law-abiding or a Communist, and if you are in Johannesburg after 10 you must be a Communist."

Oct. 24: Jacob Mahsbene is buried in Soweto. Thousands of people attend, including student activists who believe that Mashabene has been murdered by the police. As they begin to sing political songs and give Black Power salutes - a motion one judge has described as a crime here and based convictions on -eight police vans appear and police open fire on the unarmed crowd.

Crouching behind a tombstone as bullets whistle around him in a fusillade that will leave six dead is one of Mashabane's friends, who decides: "They are trying to kill our very spirit. They will shoot us on sight now."

Since Oct.24, there have been no political demonstration in Soweto.