(Adds quote from Energy and Mines Minister, updates copper prices, adds details on labor ministry authorization, adds background on previous nationwide strike)
LIMA, July 19 (Reuters) - Unionized workers at mines in Peru, the world's second-biggest copper producer, started an indefinite, nationwide strike on Wednesday to protest the government's proposed labor reforms, the head of a federation of mining unions said on Wednesday.
Workers at 56 mining unions in the Andean country, including the country's top copper mines, are taking part in the strike, said Ricardo Juarez, the head of the National Federation of Mining, Metallurgical and Steel Workers of Peru (FNTMMSP).
Juarez told Reuters the stoppage will likely curb copper production at some of the country's largest mines, including BHP Billiton Plc and Glencore Plc's Antamina, Freeport-McMoRan Inc's Cerro Verde, MMG Ltd's Las Bambas, and Southern Copper Corp's Cuajone and Toquepala deposits.
However, a spokesman for Peru's main mining association, the National Society of Mining, Oil and Energy, said mining companies have contingency plans in place that should prevent any major impacts on production.
"The effect on mining production so far today is relatively limited," Energy and Mines Minister Gonzalo Tamayo said in a local radio interview on Wednesday morning. "The companies have contingency teams, they have some scenarios predicted."
Three-month copper prices on the London Metal Exchange were down 0.1 percent on Wednesday at $5999.50 per tonne after the start of the strike, which followed the metal's recent rally in the wake of strong Chinese economic data.
Peru is also the world's second-biggest zinc and silver producer, fourth-biggest lead producer and sixth-biggest gold and tin producer, according to the energy and mines ministry.
Juarez said miners want President Pedro Pablo Kuczynski to drop proposed labor reforms that Juarez said would loosen safety rules, make firing workers easier, and shift the burden of paying into an unemployment fund to workers from employers.
Peru's labor ministry has rejected the mining unions' request to authorize the strike, but an appeals process will likely give workers several days to continue with the stoppage before companies can hire new workers. Peru's labor ministry has not responded to requests for comment in recent days and could not be reached before normal business hours on Wednesday.
Workers at the federation, an umbrella group for hundreds of unions, had first voted to strike in May. A nationwide strike two years ago had little impact on production as companies had contingency plans in place. (Reporting by Teresa Cespedes, Mitra Taj and Marco Aquino; Writing by Mitra Taj and Luc Cohen; Editing by Bernard Orr)