The "Sun City Express" sets out daily from downtown Johannesburg carrying passengers to a country the world says does not exist where they can enjoy pleasures for which they would be arrested in South Africa.

Two hours later, the caravan of buses stops in the placid rural setting of the Pilanesburg Mountains and deposits its riders in front of a cavernous hotel lobby. Slot machines are clanking, flashing and sometimes spitting out coins. Black and white chorus girls bob and swirl their bare breasts in the dancing show.

Croupiers imported from Europe spin roulette wheels and blackjack is a major draw in the mirrored, hushed casino where people of all races casually mix as they do also in the restaurants, discos, swimming pool, sauna and, if desired, in bed.

By Las Vegas standards, Sun City offers meat-and-potato fare. But for South Africans who live under the puritanical ethic of the ruling Afrikaner elite that forbids such activities, the casino resort is a treat. In its multiracial aspect, it is possibly a taste of the future as well.

Partly because of its novelty, and partly because of an economic boom, southern Africa's largest resort, casino and sports complex has had a 90 percent occupancy rate in its rooms since it opened just over a year ago. On an average weekend, 15,000 South Africans come as day-trippers by car, bus, and plane to see the 120,000-acre game park, the Gary Player-designed golf course, the tennis courts and to play out some of the recreational fantasies they cannot enjoy at home.

Fantasy is integral to Sun City. The $42 million entertainment emporium is built in a place called Bophuthatswana. On maps made in South Africa, Bophuthatswana consists of six unconnected parcels of rural countryside northwest of Johannesburg that are the homeland or reserve for South Africa's 2 million Tswana people.

In December 1977 this homeland was made an "Independent" national state under Pretoria's apartheid system, depriving the Tswanas of their South African citizenship and political rights in greater South Africa. Although no other country recognizes its independence, the South African government considers Bophuthatswana another country.

So even though one never shows a passport or crosses an international border post on the drive to Sun City, here one can gamble, ogle bare breasts and mix races without a visit from police. Casinos and strip shows have existed for several years in neighboring black-ruled states, but Sun City is the first built in what is internationally regarded as territorial South Africa.

Although only a minority of blacks can afford the luxury of even a day trip to Sun City, those who come say they enjoy being treated like a normal customer without the constraints of South Africa's segreagationist policies. "It actually makes a case for the homelands," one black, a vehement critic of the homeland policy, said wryly as he gazed across the grounds.

Among the patrons of Sun City are many Afrikaners, whose presence highlights a cultural hypocrisy that irks many South Africans who must live under their mores forbidding movies and televised sporting events on Sundays.

"It's because we have these people here in South Africa called the Afrikaners," Val Day replied slowly when asked why Sun City had to be in Bophuthatswana. "They're religious fanatics. To them, gambling is against their religion," she said as she prepared to board the "Sun City Express."

One of the bus drivers is Afrikaner Pieter Senekal, who does not blame his people but rather "the priests and ministers -- we've got too many of them on our backs." The South African government would in fact like to have Sun City for itself, he added, because then "the money would be coming their way."

As it is, the money is going to the 50-50 partners in Sun City: the Bophuthatswana government through its National Development Corp. and Southern Suns Hotels, the largest hotel chain in this part of the world.

Both partners refuse to say how much they are earning from Sun City, for which they each initially put up $5.2 million, borrowing the rest to build the resort. But clearly, both are satisfied.

Some who look at Sun City seen ostentatious, tawdry extension of the wealthy white lifestyle and economy that dominates this region in which there are extremes of black poverty. Its glitter, tinsel and good-time aura are in stark contrast to Bophuthatswana's nearby poverty-ridden black slums of Garankuwa and Winterveldt. For them, the use of development funds on Sun City rather than on rural development is an example of misguided capitalism.

The development corporation director, Wynand van Graan, disputes this. "I say we could not have done better," he said in an interview.

Sun City has stimulated the tourist industry in Bophuthatswana, it has attracted other business investments and it has created 1,700 jobs for local blacks, Van Graan said. In this latter achievement, Sun City fits in with Pretoria's policy of creating alternated economic "growth points" in rural areas to slow black migration to the cities.

In addition, Bophuthatswana levies a 35 percent corporate tax on hotel and casino earnings.

An expansion is under way that will include an 8,000-seat conference center and entertainment center. But getting stars to come to this stepchild state may not be easy because of international hostility to apartheid. A few months ago Southern Sun's chairman, Sol Kerzner, had to make an embarrassing cancellation of a match in Sun City between Bjorn Borg and John McEnroe after antiapartheid activists pressured the tennis players to bow out.