UPDATE 4-Peru miners start nationwide strike, gov't sees little impact

(Adds new comment from mining association to fourth paragraph, closing copper price to seventh paragraph)

LIMA, July 19 (Reuters) - Unionized workers at mines in Peru, the world's second biggest copper producer, started a 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 top copper mines, are striking, said Ricardo Juarez, 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's and Glencore Plc's Antamina, Freeport-McMoRan Inc's Cerro Verde 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 the vast majority of workers at medium and large mining companies went to work and production was not likely to be significantly affected. Some 80 percent of mining workers have put down their tools, Juarez said.

"The effect on mining production so far today is relatively limited," Energy and Mines Minister Gonzalo Tamayo said in a radio interview on Wednesday morning. "The companies have contingency teams, they have some scenarios predicted."

Juarez said miners want President Pedro Pablo Kuczynski to drop proposed labor reforms that the union leader said would loosen safety rules, make it easier to fire workers and shift the burden of paying into an unemployment fund to workers from employers.

Southern Copper, owned by Grupo Mexico, said its divisions were operating at 100 percent capacity despite the strike.

The strike did not appear to pressure copper prices, which ended 0.2 percent lower on the London Metal Exchange after a recent rally on 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.

Peru's labor ministry has rejected the 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.

The labor ministry did not immediately respond to a request for comment on Wednesday.

Workers at the federation, an umbrella group for hundreds of unions, first voted to strike in May. A nationwide strike two years ago barely affected production. (Reporting by Teresa Cespedes, Mitra Taj and Marco Aquino; Writing by Mitra Taj and Luc Cohen; Editing by Bernard Orr, Meredith Mazzilli and Jeffrey Benkoe)