Workers at the world's largest privately owned copper mine in northern Chile went on strike Monday to press their demand for better pay, and by midday production was down by 60 percent, a company official reported.

Union spokesman Pedro Marin said workers were gathering at a plaza in Antofagasta, 1,600 kilometers (995 miles) north of Santiago, for a planned march.

Other miners blocked an access road to the mine with rocks and parked buses.

A union assembly was scheduled for late Monday and a vote on the company's contract proposal was likely, Marin said. The proposal was rejected by the union leadership.

The company has called its contract offer, made in government-mediated talks, final. It includes a 3 percent salary increase and one-time bonus of US$17,000 (€13,200).

The workers are asking for a 13 percent wage increase and a bonus of US$21,190 (€18,400).

There was no immediate comment by the company on Monday's work stoppage but it said earlier that it would implement a "contingency plan." No details of the plan were announced, but Marin said it includes hiring around 1,000 outside workers and contractors to maintain some production.

Around noon Monday, a company spokesman, Mauro Valdes, told the Santiago daily El Mercurio that production had dropped by around 60 percent.

Escondida produces around 3.6 metric tons (4 tons) a day, or around one quarter of Chile's total output. Chile is the world's largest copper exporter.

The Australian-British consortium BHP Billiton PLC owns 57.5 percent of the mine, while Rio Tinto PLC, also Australian-British, holds 30 percent, and the Mitsubishi Corp.-led Japanese consortium 10 percent.