At least two people were killed in Peru when police used tear gas to put down a violent demonstration against Swiss-owned Xstrata staged by striking miners and townspeople, officials said.

Prime Minister Oscar Valdes declared a state of emergency late Monday, saying it was needed to "safeguard public security and allow free transit" after the two were killed and a provincial prosecutor was briefly kidnapped.

Valdes said at a press conference in Lima that the 30-day decree limiting personal freedoms was necessary because "this is not a peaceful demonstration. It is extremists, who are attacking the police."

Fifty people were injured on Monday, including 30 police, according to Interior Minister Wilver Calle.

Hundreds of demonstrators had lugged tree trunks and boulders into roads in the Espinar area of Cusco in southern Peru to protest what they say is the mining company's pollution of the Salado and Canipia Rivers.

They are demanding environmental studies, citing local animal deaths. They also want the company to increase from three to 30 percent the amount of earnings it invests in local economic development.

It was the eighth day of demonstrations that have left at least 70 people injured, officials said.

Activist Herbet Huaman, of the Front for the Defense of Espinar, said 42 protesters have been wounded, with some in serious condition after being shot by police.

Prosecutors have filed charges against 58 people, including local officials and union leaders.

Swiss-based mining giant Xstrata said its environmental operations complied with all legal requirements but it was willing to undertake extra monitoring "to clarify any perceptions or concerns that the population may have."

"We are deeply distressed by yesterday's violent protests," said a spokesman of the firm based in central Switzerland's Zug.

"We do not condone violence in any form and we are committed to constructive, peaceful engagement with all our stakeholders to address their concerns.

"We are working with national and provincial authorities to establish a dialogue with those involved in the protests to find a resolution and restore peace and order in the province."

Xstrata meanwhile said it had already expressed a willingness to improve on the agreement under which the Tintaya mine invests three percent of earnings locally.

This amounted to more than $10 million (8 million euros) in 2011, the company said.