(Adds comment from official in labor ministry to 8th and 9th paragraphs)
By Teresa Cespedes and Marco Aquino
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 has likely curbed 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 output would not likely be significantly affected.
Southern Copper, owned by Grupo Mexico, said its divisions were operating at 100 percent despite the strike.
"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.
Juan Carlos Gutierrez, the general director of Peru's labor ministry, denied that potential changes to labor laws under evaluation now in Kuczynski's cabinet would weaken workers' rights.
Gutierrez said the labor ministry has until Friday to decide whether to declare the miners' strike illegal, a move that would force them to return to work or risk losing their jobs.
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.
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 and Marco Aquino; Writing by Mitra Taj and Luc Cohen; Editing by Bernard Orr, Meredith Mazzilli, Jeffrey Benkoe and Diane Craft