Customers fill the narrow, dirt-tracked aisles of the Cab supermarket in Zwide, one of this city's sprawling black townships. Shelves are jammed with goods, and 20-pound bags of potatoes and cornmeal are stacked outside the door. Asked how much business has increased recently, owner Alfred Makwela just smiled and said, "It's a secret."

About a mile away at the Springbok Butchery, a spotless, ultramodern grocery store in the white-owned shopping district known as Sidwell, the aisles are vacant. Employes loiter around unused electronic cash registers. "You can see for yourself -- it's empty," said the manager, who refused to give his name.

The striking contrast illustrates the impact of the newest and possibly most effective weapon South Africa's disenfranchised black majority is wielding against the country's white-minority government -- its buying power. For the last few weeks, blacks in the eastern Cape region, led by local organizations affiliated with the antiapartheid United Democratic Front, have begun boycotting all nonblack stores in their communities.

The boycott has dealt a body blow to white businesses in more than a half dozen cities and towns, including Grahamstown, Queenstown, Cradock, Uitenhage, Port Alfred and -- beginning 10 days ago -- Port Elizabeth. Many analysts believe the protests are one of the reasons the government declared a state of emergency in most of this region last weekend.

The proclamation appears so far to have had no impact in bringing the boycotts to a halt. If anything, some people in Zwide say, the new crackdown has only made residents angrier and more determined to stay away from white commerce. There is also talk of spreading the boycotts to other parts of the country to protest the emergency.

The subsequent roundup by police of dozens of activists has also created a dilemma for white business leaders, who are seeking to negotiate an end to the boycott. Now that they are ready to talk, they find they have no one to talk to.

"We felt towards the end of last week we were really getting somewhere, although things were at a delicate stage," said Tony Gilson, director of the city's Chamber of Commerce, as he described talks his group is conducting with boycott leaders. "There's no question that the state of emergency has jeopardized those negotiations."

Boycotts in fact have a long tradition in South Africa and reached their peak during the 1950s when they were regularly used in an attempt to wrest concessions from the government. They fell out of favor because of their lack of success.

Their return here is partly a recognition of the growing buying power of the black community, which two years ago for the first time accounted for more than half this country's retail sales. It also is a byproduct of the rise of a new set of grassroots community organizations that appear to have the discipline and support to make these measures stick.

Sometimes the boycotts appear spontaneously, as when residents of the town of Cradock stopped frequenting white-owned stores late last month to protest the mysterious murders of four black activists there. At other times, the measures are planned well ahead.

In Port Elizabeth, community organizers laid the groundwork three weeks early, drawing up a list of demands, holding small meetings to discuss the plans and warning black businesses to increase their stocks. The businessmen were also warned to lower their prices and not gouge customers during the action.

The demands here include the withdrawal of the Army from the townships, an end to the mysterious "disappearances" of local black leaders -- which many here blame on the police -- and a lifting of the government's ban on political meetings. They also want the government to scrap the black local authorities act that set up nominally self-governing township councils that opponents contend are occupied by black "collaborators."

"Our intention is not revenge or to destroy the retailers in town, but to drive home to the state the importance of these issues," said Mkhuseli Jack, spokesman for the boycott committee. "The boycott, as far as we are concerned, is the only way we can express our views."

White businessmen are quick to point out they cannot directly meet any of these demands, but Jack said he expected the business community to use "its powerful influence" on the government.

The boycott's impact so far has been devastating on this city, which has already been hard hit by South Africa's lingering economic recession. The head of the local traders association warned that half of the city's small shops may be forced to close if the action continues. A survey by the opposition Federal Progressive Party here last week showed business losses ranging from 30 to 100 percent.

One of the methods by which the boycott is enforced is intimidation, many here say. Boycott activists attempt to monitor white stores or observe buses returning to the townships from white areas, looking for people with packages. "If they catch you, they'll burn your house," said a black businessman. "It has happened here several times."

Ebeneezer Makina, an activist in the Azanian People's Organization, the UDF's major rival here, said his group does not support the boycott because it believes such measures hurt the black community economically without compelling real change from the government. He pointed out that blacks will lose jobs if white businesses begin to fold. Nonetheless, he said his organization has kept "a low profile" on the issue because the boycott is being widely adhered to in the townships.

Jack did not deny intimidation has taken place but said his committee set up a special group to monitor and prevent it. As of last week, he said, it had received no complaints.

"Every black who is suffering knows the boycott has a purpose, and its purpose is to help us," said a black high school teacher in Zwide, who asked not to be identified by name.

There has been a white backlash. Some white shop owners have suggested the government shut off food supplies or close township businesses -- powers it can wield under the emergency declaration. Many businesses have begun laying off some of their black workers.

Business spokesmen generally have taken a conciliatory approach. "We're willing to talk with anybody," Gilson said. "We know many of the grievances are things we can't handle, but we're willing to play the role of honest broker."

The problem now is that the business community can't find anyone to talk to. Some of the boycott leaders are in jail, arrested since the emergency took effect Sunday.