The South African government informed local newspaper editors and foreign correspondents today that it has decided to ban television, radio and photographic news coverage of black unrest in designated emergency areas and warned that journalists apprehended in these areas could be subject to long prison terms and fines.

The new press rules would be the latest step in an official campaign to rein in the news media. They were due to be announced publicly tonight but were delayed without official explanation, leaving open the possibility that the government will reconsider.

If enacted, the rules would severely curtail press coverage of the protests against the government's racial segregation policies and the violence growing out of those protests that has claimed more than 800 lives in the past 14 months.

Besides banning all camera and radio crews from the areas of unrest, the new rules would require print reporters, such as those working for newspapers or magazines, to obtain permission from local police before entering emergency areas. Currently, reporters are allowed to enter such areas at their own risk except in localities where specific, written regulations forbid them to do so.

Officials of the correspondents association were informed of the press ban by a senior government official at an off-the-record briefing this morning. He informed South African newspaper editors of the ban at a similar session this afternoon. The official told both groups that the ban would be announced tonight, subject to a midnight embargo.

But the announcement was postponed late tonight because of technical difficulties in obtaining the necessary government signatures, press sources said. By then several British newspapers, faced with earlier deadlines than their American counterparts, already had published news of the ban, breaking the embargo.

Earlier, the Foreign Correspondents Association here issued a statement branding as "a smear campaign" allegations by the government and some South African journalists that foreign journalists have abetted the violence and are responsible for its continuation.

In a further extension of its powers, the government granted nationwide immunity from prosecution to policemen and soldiers for actions taken under its three-month-old state of emergency decree. Previously the indemnification had applied only to security forces in the 38 districts designated as emergency areas. Those districts cover about one-third of the country's population and include three major South African cities.

Police also announced 473 new arrests this week under the emergency, bringing to 5,876 the number detained since the decree took effect, 1,152 of whom are still in custody.

Journalists caught violating the new rules could be subject to the same penalties South African citizens face under the emergency decree: immediate arrest and indefinite detention without charges or access to an attorney. If they were later tried and convicted of violating the rules, they could receive prison terms of up to 10 years and fines of $4,000.

British Foreign Secretary Geoffrey Howe spoke out in a television interview against such a press ban, Reuter reported from London. "Action of this kind does not deal with the substantive causes of the problem in South Africa," he said.

The official campaign against foreign journalists began in September with an article in the Johannesburg Star newspaper quoting unnamed government sources as contending that unidentified foreign television crews had paid black students to burn schoolbooks on camera and had asked a mob of blacks to reenact a stone-throwing incident.

The allegations in the article, which accused foreign reporters of "frequently transgressing basic principles and ethics of journalism," have been cited repeatedly by officials in their own attacks on the foreign press, including a speech to correspondents by President Pieter W. Botha last night. Officials have not responded to repeated requests from journalists that they identify the TV crews allegedly involved and provide evidence to support the accusations.

In its statement, the correspondents' association condemned what it called "vague, unsubstantiated accusations that are sometimes libelous, sometimes factually incorrect and always directed against unnamed persons." It said the government should pursue its allegations in court, "otherwise it appears the authorities and those local journalists who support them are simply looking for scapegoats."

As part of their continuing crackdown on press coverage, police briefly detained a two-man news team from the Cape Times newspaper in a township outside Cape Town today, confiscating four rolls of blank film.

A two-man CBS camera crew was acquitted in Cape Town on charges of failing to obey a police order to leave the campus of the University of the Western Cape during a protest rally Oct. 22.

Police and soldiers put on a massive show of force in downtown Johannesburg today, forestalling repetition of incidents of black rioting that rocked the city on two previous Fridays. A bomb exploded in a Johannesburg office building later in the day; no injuries were reported.

Hundreds of police and soldiers, armed with machine guns, rifles and batons, patrolled the streets of central Johannesburg throughout the day. Other soldiers manned roadblocks at main roads leading to Johannesburg from Soweto, the sprawling black urban center, searching vehicles for weapons.

In Cape Town, scene of two months of mounting violence that has begun to spill into white areas, motorists in a white neighborhood narrowly escaped injury last night when a gasoline bomb was thrown onto a main freeway from a pedestrian bridge overhead. Police today announced a large-scale security sweep in two mixed-race townships that they said resulted in "a number" of arrests.