Early this week, a group of Christmas shoppers clustered around a cardboard box that had been left on a busy downtown street.

"We're waiting to see if it explodes," one onlooker declared.

Fortunately for them, it didn't. But their behavior indicated that not many South Africans are paying much attention yet to their government's warnings about the emerging phenomenon of urban terrorism.

Since a rash of bombings began Nov. 24, the government has been warning South Africans that, lucky as they have been up to now, they may have to live with a dramatic upsurge in bombings and bomb scares in the future.

"In an urban terror situation such as that now developing in this country, no one - no one at all, black or white - is immune," proclaimed the pro-government newspaper, The Citizen.

The daily paper, with the coopeation of the South African security police, recently ran a four-part series designed to make the public "bomb conscious."

The articles included tips for telephone operators on handling bomb threats, instructions for tenants in buildings that have received threats and a new rule of etiquette:

"South Africans must learn it is now very bad manners to leave unattended articles lying about," said Brigadier C. F. Zietsman, head of the security police.

Officially the government does not consider the bombings specific protests against its racial policies, but rather part of worldwide phenomenon, or as one security association president put it, "A Third World unpleasantness," that has belatedly come to affect this tense country.

Many residents here agree with this attitude. "It's a worldwide way in which warped people express political dissent," said S. Nigel Mandy, manager of the Carlton shopping center. "In this city, it might be against the government . . . but I don't regard it as a demonstration against apartheid. In Spain, the Basques bomb in order to get apartheid."

Shortly after the first bombing in the last week of November the government-run radio reminded its listeners that urgan terrorism is a fact of life in much of the world - Northern Ireland, for example - and that in some countries it is not possible to walk down the street safely.

"Remember that as you walk the quiet streets of Johannesburg," the commentator said.

Other people especially critics of the government would interpret the bombings as anti-apartheid acts, but no one knows the bombers' motives for sure because they have failed to follow up their violence with anonymous phone calls claiming responsibility, as terrorists have done in other countries.

Some political observers suggest that the bombings might be the work of students who fled South Africa over the past 18 months and who were recruited abroad for urban terrorist training by the banned African National Congress.

Police officials were either reluctant to answer reporters' queries on the bombings, or failed to return telephone calls, but Brigadier Zietsman gave support to the student theory in a local newspaper interview when he said: "Over the last year police have picked up steadily increasing numbers of 'shopping bag' or 'suitcase' bombs . . . Combined with the increasing flow of Sowetan students returning from terrorist training. This makes for a high-risk bomb situation." Sowetan students are from Soweto, the black township near Johannesburg.

Coinciding with the bombings has been a series of court cases in which several young men have been charged with possession of explosives or with recruiting others to go abroad for military training.

Minister of police and justice, Jimmy Kruger, has said that the bombings are proof that his Oct. 19 security clampdown, in which dissident organizations were banned and scores of people were arrested, had been necessary.

In a related announcement made last week, Maj. Gen. David Kriel, deputy commissioner of police in charge of riot control, said that violence, arson, rioting and unrest in South Africa's black townships had dropped by up to 85 per cent since the Oct. 19 clampdown.

The four recent bombing began with the explsoion in the post downtown Carlton shopping center, owned by diamond magnate Harry Oppenheirmer's Angle-American Corp. In contrast to that blast, the three subsequent bomb targets - a railway van, the storeroom of a suburban police station and cars in a railway parking lot - were not well-known or in crowded areas.

Most of the victims were injured only slightly by flying glass and it appears that the bombs were not meant to kill or maim as much as they were meant to destablize South African society and frighten people.

Surveying the damage at the most recent blast site, Brigadier J. B. Wiese, divisional commander of police for the East Rand. said the bombs "have all been let off at relatively quiet periods of the day and it seems as if these people just want to cause panic and not injure anyone. May message is, don't panic."

Johannesburg residents appear to be taking the bombings in stride with a certain kind of nonchalance and fatalism that indicates that they believe this urban violence was inevitable.

The white population was much more shaken up earlier this year when two young black men armed with machine guns ran into a downtown garage and shot two men dead. Only a few months before, the Carlton center had been the target of bomb attempt when a man tried to throw a grenade into a restaurant and blew off his hand instead. Kruger referred to those two events as the beginning of urban terrorism in South Africa.