CARACAS, Venezuela (AP) _ As tensions mounted between President Hugo Chavez and opposition labor groups, tens of thousands of oil workers went on strike Wednesday to demand higher wages.

Oil workers union leader Vladimir Blanco claimed the strike brought 90 percent of oil production and refining activity to a standstill in Venezuela, which exports about 3 million barrels daily and is one of the largest suppliers of crude oil to the United States.

Oil is Venezuela's largest foreign currency earner.

State-owned Petroleos de Venezuela S.A. insisted the strike by about 25,000 of Venezuela's 40,000 oil workers didn't immediately affect crude exports or production, and said it would be able to supply crude and derived products for at least a week under strike conditions.

Managers were replacing workers in many jobs and automated operations helped minimize the strike's impact, vice president Domingo Marsicobetre said.

Government officials were to meet representatives of Fedepetrol, but union leader Rafael Zanbrano said he was skeptical an accord could be reached.

``We have been talking for the past 11 months and nothing has been resolved, so why should we talk now?'' he said.

Meanwhile, National Guard troops were patrolling Venezuela's major oil installations to prevent violence.

Wednesday's strike highlighted the growing conflict between opposition labor leaders and Chavez, who has denounced union leaders as corrupt.

Fedepetrol has been without a labor agreement since November. Workers went on strike in March to protest a government order to suspend negotiations for six months. That strike had no major effect on oil production.

Fedepetrol is seeking a raise of $9 daily. The monopoly has offered a raise of $5 a day. Oil workers' minimum wage is about $14 a day.

The monopoly insists the workers' proposal would cost it an unacceptable $2.9 billion over two years.

Frustrated by months of fruitless efforts to come up with a collective bargaining agreement, the oil union had threatened to strike during a Sept. 26-28 Organization of Petroleum Exporting Countries summit in Caracas.

The union followed through after Congress, which is dominated by Chavez's coalition, this week called a Dec. 3 national referendum for a plan that would scrap the country's four main opposition-friendly labor unions. They would be replaced by one central organization, with the leaders elected later.

Labor leaders say the plan is unconstitutional because the referendum would allow outside parties to make decisions on the internal affairs of private organizations and would violate a constitutional right to organize.

The Geneva-based International Labor Organization also has protested the plan.

``No one can force a citizen to belong to a single political party, nor a worker to belong to a single union,'' said Carlos Ortega, director general of the Confederation of Venezuelan Workers, Venezuela's largest umbrella labor organization.