(Adds details about strike, background)
SANTIAGO, June 15 (Reuters) - Union No.1 at Chile's Collahuasi copper mine said its workers had put down their tools for a 24-hour strike Monday morning, while the company said it had implemented a contingency plan to safeguard workers' security.
Collahuasi, one of the world's largest copper mines, is a partnership between Glencore Plc and Anglo American Plc .
The strike kicked off at 8:00 a.m. local time (1100 GMT) and "has a high turnout," the union said on its website.
Collahuasi said it "rejects this measure, which it believes is not justified because the issues raised by the union have already been addressed."
In 2014, Collahuasi produced around 470,400 tonnes of copper, or nearly 8 percent of top producer Chile's total output.
Chile, which produces a third of the world's copper, is struggling with dwindling ore grades in many of its aging deposits, although new mines and recoveries in troubled ones are pushing up overall output. (Reporting by Fabian Cambero; Writing by Anthony Esposito; Editing by Chizu Nomiyama and Bernadette Baum)