It began with a murky labor dispute at a South African coal mine: The death of a worker in a dump truck accident led to a strike by miners over their right to hold a memorial service. As tensions escalated, private security forces using tear gas and rubber bullets were called in and 129 workers were fired by management.

That February 1985 incident at South Africa's Rietspruit mine, half SOUTH AFRICAINTERNATIONAL PRESSURESof which was owned by Royal Dutch Shell, was the spark for what has turned into an international boycott of Shell spearheaded by a coalition of American labor and antiapartheid groups demanding that the company pull out of South Africa. In the United States, the campaign has produced picketing in front of Shell gas stations in more than 20 cities, demonstrations in front of Shell corporate offices and the cutting up of hundreds of Shell credit cards.

In Europe, where boycott activity has spread to seven nations, some of the consequences have been more disruptive: On two occasions last spring, gasoline fire bombs exploded at Shell stations in the Netherlands, and there have been reports of vandalism at a handful of other Shell stations. On Nov. 15, during an international "day of action" against Shell, a company service station in Denmark had its gas hoses cut and sugar dumped into its gas tanks.

The Shell boycott is one of the more dramatic examples of the escalating pressures that have faced most multinational corporations operating in South Africa. The extent to which those pressures have spread across the Atlantic was underscored last week when Britain's Barclays Bank announced it will be leaving the country, following the lead of such major U.S. firms as General Motors Corp. and International Business Machines Corp.

But there is another dimension to the debate over Shell that has prompted antiapartheid activists to target the company. Royal Dutch Shell, a giant Dutch-British concern with headquarters in London and The Hague, is the oil company with the largest and most extensive investments in South Africa. As a result, they say, Shell's operations there symbolize one of the most sensitive issues in the debate over South African disinvesment: the crucial, and secretive, role that international oil plays in fueling the South African economy.

"Shell petroleum is in the vehicles that lumber into the black townships to bring the soldiers that are arresting and killing people," charges Randall Robinson, executive director of TransAfrica and one of the organizers of the Shell boycott. "It plays a pivotal and strategic role in propping up the South African government."

So far, Shell officials are playing down the boycott, saying there is no evidence that it is catching on or hurting the company's overall sales. Yet there are growing signs that the campaign has rattled the world's second-largest oil company.

In South Africa, Shell executives have talked recently about the international "onslaught" facing the company. At least partly in response, Shell South Africa Chairman John R. Wilson has become increasingly outspoken in his public denunciations of apartheid and South African government policies, embroiling him at one point in a dispute with President P.W. Botha that made front-page headlines.

And in the United States, company officials are openly expressing fears that the boycott might escalate and hurt the independent dealers and jobbers who operate about 11,500 Shell gas stations. At least two of those dealers, one in West Virginia and another in southern Illinois, have dropped their affiliation with Shell over the protests.

"The boycott is a concern to Shell," said Tony Cimino, a spokesman at Shell U.S. headquarters in Houston. "This could adversely affect the business of our dealers and our jobbers. They're the ones who see the picketers and have the customers turned away . . . and they've done nothing to deserve this happening to them."

The campaign against the company was born in January after the predominantly black National Union of Mineworkers in South Africa issued a call for help to its international affiliates over the Rietspruit coal mine incident. But the idea for a worldwide boycott against Shell -- strongly pushed by United Mine Workers President Richard Trumka, who also has battled Shell over lingering contract disagreements in this country -- immediately appealed to antiapartheid activists because of the company's preeminent position in the South African oil market.

In fact, Shell is only one of several international oil companies that operate in South Africa (Mobil, Caltex, British Petroleum and French-based Total are the others). But Shell, with an estimated $ 400 million in assets and more than 2,500 employes, is the biggest and most entrenched. Shell South Africa co-owns the largest oil refinery, co-manages the largest offshore oil import facility, operates a major oil pipeline, owns 853 gas stations and has extensive investments in chemical and asphalt plants as well as coal, lead and zinc mines.

Shell officials in London and The Hague -- the company's dual headquarters -- contend that acceding to the boycotters' demand for a pullout would have no effect on the South African government. Shell's oil facilities would simply continue to operate under different ownership, they say.

"In our view, disinvestment would be nothing more than an empty gesture," said Michael Herbert, the director of Shell external affairs, in a telephone interview from London. "It would result in nothing more than the replacement of the Shell emblem with somebody else's -- and that somebody else might not be as enlightened an employer as we like to think we are."

However, Shell officials acknowledge that the boycotters' focus on the supply of oil has placed them in an awkward position. South Africa has virtually no indigenous crude oil and Shell, as with all the major oil companies, resolutely insists that it does not import crude petroleum into the country -- a restriction imposed by all OPEC nations as well as the U.N. General Assembly, which has been urging an international oil embargo on South Africa since 1963.

That leaves open the question of where and how Shell South Africa obtains the oil that it refines and markets in the country. The company says it can't say. The reason: an array of South African laws that strictly prohibit domestic companies from disclosing, even to their corporate parents, any information about the country's oil supplies or whom they sell to.

In a recent interview with an in-house company magazine, Royal Dutch Shell President Lo van Wachem said Shell South Africa must "abide by the laws of that country." As a result, he added, he could say only that the South African subsidiary gets its oil from unspecified "third parties," adding that "we . . . do not know from whom or from where the oil comes."

"Nobody in this office or in The Hague knows where the oil comes from," Herbert said. "Obviously, Shell South Africa knows, but they are not allowed to tell us. . . . I realize that doesn't sound very credible and this is one of the problems we face. But it does happen to be true."

Such comments have been greeted with varying degrees of disbelief and derision by the company's critics. "I find it preposterous that a corporation the size of Royal Dutch Shell does not know where one of its wholly owned subsidiaries procures its oil," said Kenneth Zinn, international representative and Shell boycott coordinator for the United Mine Workers. "They [Royal Dutch Shell] are responsible for the actions of their company."

Meanwhile, Shell officials counter that there is more to the boycott against them than the oil issue. In particular, they say, the campaign has as much to do with union politics in the United States as it does with South African apartheid.

One of the key organizers of the Shell boycott is Richard Trumka, president of the UMW, which has been engaged in a longstanding dispute with the A.T. Massey Coal Co. in Richmond, half-owned by Shell U.S. Massey was the only major coal company to refuse to sign a 1984 wage agreement negotiated by the UMW with Bituminous Coal Operators Association, resulting in a 15-month strike against some of the company's mines. The strike was called off by Trumka last December and the dispute over the wage agreement is now in the courts.

Massey officials note that the Shell boycott was organized by Trumka in January, one month after the end of the strike against Massey. "People can draw their own conclusions," said Paul Barbery, general counsel of Massey. "But you have to assume that part of their [the UMW's] problems with Royal Dutch Shell is because of their unhappiness with A.T. Massey."

Trumka was unavailable for comment on this issue, according to his press spokesman. UMW officials contend that the Massey dispute is unrelated to the Shell boycott. The officials do acknowledge, however, that the union was interested in singling out Shell because of the Rietspruit incident and the company's role in exporting what the union considers "slave labor" coal into the United States, an issue that became moot last month when the Congress passed economic sanctions forbidding further coal imports from South Africa.

Whatever the issues behind the boycott, it appears to have spurred Shell officials in South Africa into talk, if not action. As president of the Federated Chamber of Industries (the South African version of the Chamber of Commerce), Shell South Africa President Wilson made headlines in July with a public statement condemning the government's "strategy of political repression and economic isolationism" -- a stance that earned him a stinging rebuke from President P.W. Botha.

And in an August speech to his senior managers, Wilson explicitly warned that Shell may well be forced to disinvest if the Botha government does not speed up political reforms, release political prisoners and negotiate with parties representing "all shades of opinion."

"Why is Shell taking a high-profile, political stance?" asked Wilson, according to a text of the speech. "The answer can be summed up in one word: survival. . . . The situation is not comfortable. Shell's position is not comfortable. The threat of disinvestment is real."