With gasoline shortages starting to bite and oil exports all but halted by a walkout in Venezuela's most vital industry, the growing opposition movement has apparently found the first effective weapon in its eight-day-old general strike aimed at pushing President Hugo Chavez from office.

No other Latin American government so relies on one industry for its financial survival, making Venezuela's $40 billion-a-year national oil company the big prize in the standoff between Chavez and his die-hard opponents. Today, as opposition leaders extended the strike for at least another day, gas stations ran dry in many parts of the country and Chavez ordered private tanker trucks commandeered in an uphill effort to maintain domestic fuel supplies. Dozens of idled tankers were at anchor offshore, in view of the country's crippled oil refineries.

Chavez has called the organized movement to remove him a fiction created by the opposition-controlled media to destabilize his government and spark a military coup d'etat like the one that briefly ousted him in April, during a previous oil strike. Although the military has been purged of many dissident officers since then, the national oil company, Petroleos de Venezuela, remains controlled by opposition executives and labor union leaders who believe the current strike has brought them closer than ever to their goal of forcing him from office.

"It continues and will achieve the objective to remove Chavez," said Carlos Ortega, head of the million-member Venezuelan Workers Confederation, the largest labor group. "The sacrifices of the people will not be in vain. We are in the final hours of an authoritarian regime. We will be in the street until he leaves."

The strike has grown to include government tax workers on Margarita Island and ship pilots in the Orinoco River, national airlines and mall owners, restaurants and retailers suffering through a devastating Christmas season. No estimates have been presented on the strike's cost, but it has fallen hard on businesses during a year in which Venezuela's $100 billion economy has shrunk an estimated 7 percent.

But since the oil industry began a work slowdown on the strike's third day, broader participation in the protest by private businessmen, merchants and workers has become largely irrelevant. Even the ongoing massive street marches that have characterized the angry political debate in this country of 23 million people for much of the past year appear to have had little bearing on whether the strike should be lifted.

Ali Rodriguez, the president of Petroleos de Venezuela, appeared on television tonight to call the strike at the company "a national disaster" that will affect food supplies in coming days.

The shutdown has also changed the dynamic of negotiations to end the standoff, forcing the government to understand that time may no longer be on its side. The talks, mediated by Cesar Gaviria, secretary general of the Organization of American States, were suspended when government negotiators left the table before the strike. But the government has now returned to discuss formally a nonbinding referendum on Chavez's administration or moving up the 2006 presidential elections.

But with their hand strengthened, opposition negotiators appear to be pushing for Chavez's immediate resignation.

"This has been trench warfare for much of the year, with neither the opposition nor the government advancing in political support," said Alfredo Keller, an independent pollster. "The oil industry has sustained the president's bureaucracy and political program. Now it is the key to the opposition's success."

Chavez, a former army colonel who led a failed coup attempt a decade ago, has viewed the national oil company as the way to bankroll the "social revolution" he promised Venezuela's impoverished majority when he was first elected four years ago. Chavez has benefited from high oil prices for much of his tenure, but his efforts to use those resources on behalf of his domestic political program have caused deep dissension within the state oil company.

Created in 1976, Petroleos de Venezuela has had its traditional measure of political independence steadily eroded under Chavez. The company provides the government with $9 billion a year, its largest single source of income, and accounts for nearly 80 percent of Venezuela's export revenue. The United States receives 1.5 million barrels of Venezuelan oil a day, 15 percent of its oil imports.

Chavez has tried, with little success, to pack the company's board with political allies and has fired its chief executives with dizzying frequency. The company has had six presidents since Chavez was first elected. And the government's decision to provide Cuba with cut-rate oil in 2000 -- discounting and partially financing 53,000 barrels a day, about a third of Cuba's consumption -- antagonized the United States and many of Chavez's early supporters.

Chavez has also made an enduring enemy of Ortega, the former oil union leader who now heads the Venezuelan Workers Confederation. Chavez tried to oust Ortega by a referendum in 2000, only to see his handpicked candidate lose to him in subsequent union elections. Ortega has been the driving force behind four national strikes against the government in the past year.

With support from the country's largest business group, Ortega called a national strike of white-collar oil executives in April to protest Chavez's appointment of five political allies to the company's board. Two days later, street protests ended with the shooting deaths of 18 people, some of them Chavez supporters. A group of senior military officers then intervened to remove Chavez from office.

Chavez returned to power several days later, and since then, he has purged more than 600 dissident military officers, some of whom now occupy an elegant public plaza where three people were killed and 29 wounded Friday when at least one gunman opened fire on demonstrators. Fearing a repeat of April's coup, Chavez brought out the military's joint chiefs for a television appearance on Saturday in which they expressed support for the government.

"This government foments violence, and we want it gone," said Hector Arabia, a tour guide waiting in a long line to fill his sport-utility vehicle with gas. "We're just waiting for the military to step in and remove him."

Petroleos de Venezuela has not been purged like the military was, and remains in the hands of the senior and mid-level executives who engineered the April protests. Chavez, who has promised a major restructuring of the company, accepted the board's resignation over the weekend, but many senior executives remain in place.

Juan Fernandez, the company's planning director and leader of the dissident executives, said today that "the government is blaming the people for what is happening right now."

"First, we are Venezuelans," Fernandez said of the oil sector, "And second, we are workers."

Despite being elected twice, Chavez has maintained that he is the victim of a small elite that enjoyed political and economic power in this country for four decades before his election. He has called the media "almost criminal" for inciting resistance to the government. Indeed, round-the-clock coverage of opposition protests and declarations has left little room for the government's view. Tonight, apparently at the direction of government party officials, crowds of Chavez supporters gathered in protest around several television stations here.

Chavez's popularity has dipped sharply since he was reelected in 2000 with 57 percent of the vote in balloting required by a constitution he helped engineer. His support declined first among the middle class, which analysts said grew concerned about his tilt toward Cuba and his populist economic program. That discontent soon reached into the lower classes.

Keller, the pollster, said his surveys show that Chavez no longer has a majority among any social class. He said Chavez's support has held firm at 36 percent for the past year, reflecting a core among the poor and very poor who continue to view him as a hero and demand that he be allowed to remain in office.

Despite a "Not in Service" sign, drivers wait in line in hope of getting some of the last fuel at a gas station in Caracas after a national strike halted supplies. For some people waiting to fill their tanks in Maracaibo, the center of Venezuela's oil industry, the lines lasted more than two hours.