Workers at state-owned Corporacion del Cobre de Chile, the world's biggest copper producer, went on strike Monday to protest a modernization plan they see as a step toward privatization.

Some 15,000 Codelco employers and 30,000 contract workers are expected to take part in the walkout, a spokesperson for the FTC union told Efe.

The work stoppage coincides with the 40th anniversary of the passage of the law that nationalized Chile's copper industry and created Codelco.

While the nationalization took place under Socialist President Salvador Allende, the military regime that ruled Chile for 17 years after a September 1973 coup chose to hang onto Codelco, using the company as a cash cow for the armed forces.

Over time, however, the minerals sector was opened to private firms, and Codelco now accounts for only a third of Chile's annual copper output of around 5.7 million tons.

That development has prompted some in the Chilean left to demand renationalization of the entire copper industry.

Codelco management says the modernization initiative is aimed at boosting production, but the union suspects the real agenda is to burden the company with debt to create a pretext for privatization.

The state-owned giant plans to invest more than $17 billion over the next five years with the goal of increasing output to 2.1 million tons a year by 2020.

Codelco produced 1.76 million tons last year, representing 11 percent of global copper output, and contributed $5.8 billion to the Chilean treasury.

Copper is the Andean nation's leading export and Chile has enjoyed a bonanza in recent years thanks to China's soaring demand for the red metal.