A four-day-old general strike called to push President Hugo Chavez from office disrupted Venezuela's vital oil industry today, and the streets of Caracas filled with rival partisan crowds, leading Chavez to warn of an imminent coup attempt and urge supporters to stand by his government.

The president's warning came after the captains of seven tankers that belong to the national oil company, Petroleos de Venezuela, dropped anchor and joined the strike -- a defiant threat to the government's main source of revenue. The tankers represent more than half of the company's fleet and about 25 percent of its total shipping capacity.

The captains refused to resume work even after Chavez warned that he intended to use military force to take over the idle craft and force them to haul their normal loads. Late this afternoon on Lake Maracaibo, witnesses said the navy boarded the tanker Pilin Leon, which was carrying 280,000 barrels of gasoline, but it was unclear whether it took control. The other ships, which Chavez said had been "pirated," apparently remained on strike.

Opposition leaders announced the strike would continue at least another day, reflecting a decision to continue a financially punishing protest for as long as it takes to force concessions from the government. The announcement further complicated negotiations to end the increasingly tense standoff and raised fears of gasoline shortages in Venezuela and cancellation of oil shipments to the United States and other foreign customers.

Chavez, who until today had dismissed the strike as a media-driven display destined to fail, said in a national address that the opposition was organizing the type of events that preceded his brief ouster in April, when street protests and an oil strike provoked a military-led coup. He called on Venezuelans "to reflect so you won't be manipulated again," even as he called his opponents "fascists" and "coup-mongers."

"There is a plan in progress to defeat the constitutional government," said Chavez, who canceled a trip to Brazil. "These are groups of subversives, groups of destabilizers. I won't say opposition, a democratic opposition, which we still do not have in this country."

The president's comments demonstrated the escalating rhetoric on both sides of a broad ideological divide that has brought Venezuela, the United States' third-largest oil supplier, to the brink of more political violence. After tacitly endorsing the failed coup carried out by senior military officers in April -- and making clear its distaste for Chavez and his class-based populism -- the Bush administration has been careful in public to urge a peaceful and legal resolution of the crisis this time, preferably through new elections.

But Chavez, a former army colonel elected twice on a leftist platform, has refused to resign or move up presidential elections scheduled for 2006, as his opponents have demanded. He must go, they say, because he has ignored the constitution, ruined the economy and allied the country with Cuba and its president, Fidel Castro.

Chavez, who retains a large following among Venezuela's poor, claims he is the victim of a privileged class of people displaced by his victory in 1998 on a pledge to bring a "social revolution" to country rich in oil but where more than half the 23 million people live in poverty.

Opposition supporters massed in the streets again today, this time to march on the oil company headquarters, as they did in the hours before the April coup. But more than a thousand Chavez supporters gathered to block the way, vowing to protect the president. Opposition organizers canceled the march, claiming without presenting evidence that government-sponsored snipers had been posted along the route.

"It's logical, given what they have been doing, that there will be violence," said Jorge Molina, a captain in Venezuela's merchant marine who gathered with hundreds of Chavez supporters waiting for the opposition marchers. "We're not armed, and we're not afraid."

It was difficult to envision how the two sides can now resolve the deadlock, which has resisted a month of negotiations mediated by Cesar Gaviria, secretary general of the Organization of American States. Opposition leaders appeared split over what they would accept from the government to end the strike, indicating that the protest could last until the diverse movement of labor unions, business groups, political parties and housewives resolves those differences.

Most of the storm clouds have gathered around Venezuela's oil industry, a point of political struggle between Chavez and company managers who accuse him of trying to squander its resources for his political program. The company, known as PDVSA, provides the government with almost $9 billion a year -- nearly half its revenue -- and the United States with 15 percent of its oil imports.

The tanker captains who joined the strike anchored in Lake Maracaibo and off Puerto La Cruz, where there is a large refinery. Some of the tankers were full of gasoline and crude. They did not try to block shipping lanes.

Venezuela's three refineries -- including the largest one in the world, on the Paraguana Peninsula -- operated at a severely reduced level today, according to sources in PDVSA. The sources said work stoppages had reduced refining capacity by 1 million barrels a day, about a third of Venezuela's output, and predicted the country will default on international oil contracts within six days. Several regions in the country were beginning to experience gasoline shortages.

Edgar Paredes, president of PDVSA's petrochemical arm, said employees would stop the protest if Chavez agreed to a nonbinding referendum on his administration. The national elections board voted this week to hold the referendum Feb. 2, but the government said it would challenge the decision in the Supreme Court.

The OAS talks remained dormant just a day after several negotiators expressed hope that a breakthrough might be near. The talks are supposed to focus on setting early elections, a solution that has been endorsed by the State Department, but the topic has yet to be formally addressed in the contentious atmosphere created by the strike. In addition, the strike appears to have placed in doubt the government's earlier willingness to return control of the Caracas police department to the opposition mayor and withdraw troops from eastern Caracas, two important opposition demands.

"What was happening yesterday was a sense of giving things on both sides that might end the strike," said a source involved in the negotiations. "Today we don't have that. I'm afraid it has really moved back from even where it was."

Unpleasantly surprised by the strike's unexpected duration, government negotiators today would not sit in the same room with the opposition. Rafael Alfonzo, the owner of a food manufacturing company and an opposition negotiator, said the government's decision not to participate was part of its continuing effort to delay discussions on elections. But he acknowledged that the government's actions during the strike have increased opposition demands.

"The strike is going to last until the president says he accepts elections before the end of April," Alfonzo said. "Dialogue has died. At every turn, the government has sought to make matters worse, gassing protesters, militarizing the city. People's expectations have changed. Maybe what was good a week ago is not good enough today."

Demonstrators opposed to President Hugo Chavez march near the headquarters of the national oil company, Petroleos de Venezuela.The tanker Morichal, whose captain supports the strike, sits at anchor in Lake Maracaibo as a coast guard boat patrols nearby. Chavez threatened to use military force against the tankers, and the navy boarded one of them.