South African color lines blur - a little. More black, Asian, and Colored South Africans are moving in to `whites only' areas. Pretoria is responding with new laws to open up some neighborhoods to all races while at the same time cracking down to preserve segregation in others. Many doubt the strategy can stem the influx into white areas.

On the Saturday afternoon about six months ago that Deepak Vallab and his wife, Khadija, decided to move into their new house, a group of well-built white men had other ideas. For when the Vallabs arrived at their new address in the Mayfair section of town, the men had formed a cordon on the sidewalk shouting, ``No coloreds in this neighborhood!'' (The Vallabs are Indians.) Mr. Vallab got the message: He gunned his motor and exited as fast as he could push his fully laden truck.

But instead of returning to his old residence, he drove around the block to find the coordinator of the Action Committee to Stop Evictions (Actstop). About 60 Actstop members quickly gathered back at the Vallabs' house. They unloaded the truck, moved everything in, and organized around-the-clock vigils for the next few days.

The whites gave up and went home.

``We want to show people that we are here to stay,'' declares Actstop activist Ebrahim Sarang. ``No matter what, we're going to protect our homes, our possessions. This is our resolve.''

It is this determination that has turned white, working-class Mayfair into a largely Indian neighborhood - illegally. For under South Africa's segregationist system, called apartheid, different racial groups must live in their designated areas. And, according to the Group Areas Act - as this law is known - most of Mayfair is for whites only.

But through economic clout and sheer force of numbers, these middle-class Indians are shaking one of apartheid's most important pillars. Their strategy is to make offers on Mayfair's modest bungalows that owners simply cannot resist, then form companies with obliging whites as major shareholders to finish the purchases. And with it, they have opened the floodgates.

While Indians here are not about to bring the whole segregationist structure down, their success underscores what many analysts see as an almost unstoppable urbanization process. For as more of South Africa's 28 million nonwhites move into their newly created middle class they will have the buying power to escape the cramped squalor of their isolated ghettos and move closer to town. Their inevitable destination: the more pleasant and plentiful places where 4.5 million whites live.

The trend is significant, analysts say, because it effectively renders useless future attempts to legislate separate urban living areas. Even now, the government is getting hammered by opposition right-wingers over the issue. Thus it is about to launch new laws to try to accommodate both the black influx to urban areas and growing white resistance to the movement.

Officials say a new bill will provide for already-integrated neighborhoods such as Mayfair and certain new ones to be ``opened'' to all races. At the same time, however, an amendment to the current Group Areas Act will allow authorities to crack down on offenders in all-white areas. But many people see it as a losing battle.

``Blacks are buying houses in Jeppestown, Judith's Paarl, Troyeville,'' says realtor Charles Owen, reeling off the names of white Johannesburg suburbs. ``And they are going to continue, no matter what the risk, because there just aren't enough houses in the townships.''

Some political scientists see potential danger in the process. They worry about rising white vigilantism over the government's inability to stem the tide. They also fear that with important municipal elections in October, a white backlash could spur Pretoria to try mass evictions.

The Indian connection to Mayfair, a rectangular piece of property west of downtown Johannesburg, goes way back. Indians had lived and worked in the area for decades before being forced to relocate to Lenasia township, some 20 miles away, when the Group Areas Act was introduced in 1949. So it made sense to them to try to sneak back into Mayfair in the late 1970s when housing shortages in Lenasia and other Indian areas became acute.

At first, white authorities evicted people almost as fast as they moved in. All that changed in 1979, when Raghu and Dhana Naidu camped out for six weeks in a tent on the sidewalk in front of a house from which they had been evicted. The protest attracted widespread attention and led to the formation of Actstop, an association of lawyers and activists.

Actstop lawyers decided they would defend every eviction - free of charge - to try to clog up the courts and paralyze the process. (People cannot be evicted while their cases are pending.) They scored a real victory in 1982, when the Supreme Court ruled in the case of Gladys Govender that blacks (non-whites) could not be evicted from white areas unless the government provided alternative accommodations.

Because there were thousands of Indians (and blacks) on waiting lists for houses in legal areas, practically no one could be evicted. ``With the Govender ruling, everyone suddenly realized the government was trapped,'' says Cassim Saloojee, Actstop president. ``It gave them the courage to take risks.''

And their cold cash found few closed hands among whites. Mr. Owen, the real estate agent, says people were willing to pay just about any price for a decent place to live. Pokey little houses valued at $15,000 were snapped up at $75,000. Owen recalls one man who showed up at a house he coveted with $10,000 in a shopping bag as incentive.

Motivated by inflated offers and, in numerous cases, the dread of living in an integrated neighborhood, about 65 percent of Mayfair's whites have sold their homes in the last few years. Many who remain say they have adjusted. Take Nicolaas Coxen, an electrician who has lived here for 35 years. ``These Indians,'' he explains, ``they all have good jobs, we don't hear them, we've got no complaints.''

But Mr. Coxen gets agitated when talk turns to the possibility of blacks moving in. ``Never,'' he says, almost shouting. ``Never. We don't want them. We don't mix with them.''

Others here remain adamant about keeping Indians out. The whites-only Residents and Ratepayers Association has reported just about every new arrival to the Group Areas Police, who have done nothing more than open files on them. Frustrated by the lack of law enforcement, some have taken matters into their own hands by forming vigilante groups to ``patrol'' their neighborhoods.

``If you open up living areas, then you have to open up schools and all other facilities,'' insists Allen McCabe. Mr. McCabe is known in these parts for rather raucous confrontations. ``What will be left of apartheid after that?''

Many political analysts say the ultra right-wing Conservative Party is going to steal the October municipal elections from President Pieter Botha's National Party, partly because of the Group Areas issue. That apparently is why the government is going to try to push through an amendment that will allow it to override the Govender ruling and evict people even without providing other accommodations.

Although Mayfair probably will escape such a move by being ``opened,'' illegal residents are taking no chances.

``Through this struggle, we are forging a sense of community,'' says Deepak Patel, a pediatrician. Dr. Patel sits in the living room of the house he bought last year, with its high, molded ceilings and gleaming hardwood floors. ``There is no way we are going to move out without a fight. We have had to live for too long in indecent circumstances. No more.''