WELKOM, SOUTH AFRICA -- At first glance, this sleepy gold-mining center in Orange Free State province seems a perfect model of urban planning -- pristine public lawns, clean, wide boulevards with not a single stoplight.

"A city of beauty and function, interlaced with parks and flanked by bird sanctuaries," a promotion brochure at the Town Hall reads. "Total harmony between man and nature."

What is lacking, though, is harmony between white man and black man. Just below the surface of its small-town calm, Welkom is seething with racial hatred, heightened black militancy and mounting white fear of the future. It has become the racial flashpoint of South Africa, the city whites and blacks are watching to see what will happen next.

Blacks, outraged that white vigilantes seek to impose an 8 p.m. curfew on blacks, have just completed their second boycott of white business in three months.

Whites have taken up arms because blacks killed two white housewives recently and attacked two white joggers. In mid-May, black miners stoned and stabbed to death two white mine officials in a bloody confrontation at a mine on the outskirts of town.

Welkom -- "Welcome" in Afrikaans -- was planned and built by the Anglo-American mining company as a model town for workers after gold was struck on a corn farm of the same name in 1946. It has the country's second highest income per capita for whites.

But the well-off white population of 58,000 is keenly aware these days that it is surrounded and outnumbered by disgruntled blacks. Next door, a few hundred yards from one of the 16 white suburbs, live thousands of poorly paid black miners, 21 to a room at the Saint Helena Mine hostel. On the other side of town is Thabong township, where 138,000 blacks live in similarly cramped quarters, often in squalor.

Since the release of black nationalist leader Nelson Mandela from prison in February, black attitudes here have changed noticeably, according to whites. Workers in town are more militant and assertive, ready to argue about the slightest issue, they say.

"You ask them to mop the floor and they argue for hours about who's going to do it," said the annoyed white manager of Indiana Spur, an American-style steak-and-salad restaurant.

Welkom's reputation as a right-wing stronghold is not new. A few years back, white extremists tarred and feathered the town's white mayor of the time, Guss Gouss, because he allowed a black taxi stand downtown. In theory, all public facilities here are open to all races.

Whites who would have fit into America's Old South flaunt pistols, shotguns and prejudices. Vigilante groups with names like "White Security" and "the Flamingos" chase blacks out of town after 8 p.m. A member, Hennie Steyn, expressed pride that they had "cleaned out the white suburbs" of unwanted blacks.

"I don't know if any of you have seen a black when he's aggressive. He's not like a white human. He's getting mad because that's the way he's been brought up," Steyn told reporters traveling with him in a White Security pickup truck on one of its nightly patrols.

Jan Bezuidenhout, owner of 30 farms and a gas station, walks around in White Security's khaki uniform with his .44 Magnum pistol slung in a holster across his massive wrestler's chest.

Bezuidenhout joked that "a good year" for him would be one in which he forced as many blacks into a tire as possible for a "necklacing" record. He was referring to a technique sometimes used by blacks against other blacks of burning to death with a gasoline-filled tire chained around the neck.

Racial jokes told at the Welkom Hotel pool hall by less assertive whites run along the same lines.

Hennie Muller, the leader of White Security, makes clear that he believes only his organization stands as a shield against hordes of blacks rampaging through the white suburbs.

Muller is owner of the Prestige Panel Beaters, a bodyshop in the industrial sector of town that serves as a gathering point for his vigilantes. The other day he was interrupted in his office by several reporters as he loaded a .38 revolver for his wife.

Muller has been trying to organize a counter-boycott of Thabong township by persuading white merchants who provide it with food to stop the supply. He said also that White Security has begun training black vigilantes to fight back against the black militants in Thabong.

Not all of Welkom's whites are of this persuasion. The jovial, teddy bear-like mayor, Wilhem Odendaal, is by white standards here an out-and-out liberal trying to salvage the city's reputation and prevent We-lkom from going up in racial flames.

He acknowledges that vigilantes "scored" with the white population by helping to maintain law and order after a freak tornado hit the town in March and caused massive destruction. They scored again when they organized patrols of the white suburbs, he said.

"I hate extremism," he remarked in an April interview, in which he expressed concern about the backward trend in race relations here, but South African mayors have little power.

Odendaal describes his job as little more than "public relations agent." However, he opened up the first lines of communication the other day by sneaking into Thabong township to try to persuade leaders there to call off their boycott.

Mlindo Mahoko, who runs an advice center in Thabong, said that "apart from the right-wingers," Welkom "is our pride. . . . The people of Thabong built it. They had the money, but we had the labor power. But we cannot live in it.

"That's why we are looking forward to a nonracial South Africa, so that if one wants to stay in town, he does; and if in the township, he does," he said.