A national strike escalated into street clashes between opposition forces and the National Guard today as Venezuela's vital oil industry became the epicenter of a political crisis over President Hugo Chavez's four-year-old administration.

During an afternoon rally at an executive office building of Petroleos de Venezuela, the state-owned oil company, National Guard troops fired tear gas at hundreds of employees and opposition supporters without warning the crowd to disperse. The group scattered in fear, giving new life to a national anti-government protest that earlier in the day had showed signs of waning into an opposition defeat. Tonight, as thousands of Venezuelans poured into the streets and blocked major highways with burning barricades, opposition leaders announced that the fourth national strike in the past year would be extended to at least a third day.

At the same time, the national elections board opened the door again for a nonbinding referendum on Chavez's rule, which diplomats hoped would refocus the struggle and remove it from the streets.

The oil company protest was held in support of Juan Fernandez, an opposition leader at the company who was robbed at gunpoint in his home the previous night in a crime interpreted by the opposition as political intimidation. Whether or not that was the motive, the swift National Guard sweep through the peaceful crowd recalled the street violence that preceded Chavez's brief ouster in April in a military-led coup.

Fernandez, the oil company's planning director who organized protests in April, said 90 percent of the company's management was striking to force either Chavez's resignation or early elections. He said the managers planned to remain on strike at least through Wednesday and warned that Venezuela, the third-largest U.S. oil supplier, could soon be forced to cut back shipments.

The national strike is the most prolonged push in months by a broad-based opposition movement hoping to force Chavez from office or move up the 2006 presidential elections.

Elected twice since 1998, Chavez has sought to bring what he terms a social revolution to Venezuela, where more than half the population of 23 million lives in poverty despite the oil company's projected $18.9 billion in revenue this year.

Many of those poor Venezuelans still view the president as a hero. But Chavez, a former army colonel who led an unsuccessful coup a decade ago, has enraged many other Venezuelans with fierce, class-based rhetoric, a populist political program and an ideological turn away from the United States and toward Cuban President Fidel Castro.

The Organization of American States is mediating talks between the government and the opposition, hoping an agreement on early elections might defuse the growing crisis. Following the government's military response today, however, opposition leaders said they would ask OAS countries to find Venezuela in violation of the organization's Inter-American Democratic Charter. The document commits all member countries to democratic government and requires economic sanctions against any country found in violation.

The oil industry supplies the government with nearly half its revenue and is the country's lifeblood. Company officials and union leaders said 85 percent of the oil sector, including production workers at installations across the country, were participating in the strike. But Labor Minister Maria Cristina Iglesias said "all basic industries are operating normally."

In a decision that was largely overlooked during the unrest, the national elections board voted a second time to allow a nonbinding referendum on Chavez's administration.

Last week, the five-member board voted 3 to 1 with one member abstaining to hold the referendum on Feb. 2, only to be overturned hours later by the Supreme Court for failing to muster the required four votes. Today, the fifth member voted in favor of the referendum, which will ask Venezuelans whether Chavez should resign immediately.

An injured woman is helped during protests by state oil company employees in Caracas. National Guard troops broke up the demonstration with tear gas.