Foes of Newmont Mining's $4.8 billion Conga mine project in Peru said on Wednesday they were practically kidnapped by counterterrorism police in a crackdown on left-wing activists by President Ollanta Humala.

Wilfredo Saavedra, leader of the Environment Defense Front of Cajamaraca, and Milton Sanchez, head of a civic association, were detained on Tuesday after addressing a congressional panel. They were held for 10 hours before being released.

Supporters said their rights were violated because police had no judicial order to detain them. Saavedra and Sanchez plan to file a legal complaint against the government.

Saavedra, who spent a decade in prison for belonging to the violent left-wing Tupac Amaru insurgency, has emerged as a high-profile leader in an environmental dispute that has tested Humala's resolve to govern as a centrist trying to help Peru's poor and attract foreign investment.

They practically kidnapped us, Saavedra said on RPP radio after he was released.

Humala, a former army officer who shed leftist rhetoric and recast himself as a moderate to win election in June, decreed a state of emergency on Sunday to break 11 days of protests that had shut roads, schools and hospitals in Cajamarca.

The special powers suspend freedom of assembly and allow the army to help police to end marches and rallies against Newmont's proposed gold mine.

An emboldened Humala has called leaders of the environmental protest intransigent for rejecting weeks of mediation efforts.

Today we have people who served time in jail for treason taking up causes they never defended and who are now free and acting like delinquents, he said on Tuesday, hours before the detentions.

Protesters say the U.S. company's mine would hurt water supplies and have demanded the government permanently cancel it. But the government has said the largest mining investment in Peruvian history would generate thousands of jobs and huge tax revenues.

The Conga project, which Newmont owns with Peruvian precious metals miner Buenaventura, is expected to produce 580,000 to 680,000 ounces of gold a year after it opens in 2014. It sits 13,800 feet (4,200 metres) high in the Andes and has reserves worth about $15 billion at current prices.

Newmont, which says its environmental plan for the mine was exhaustively researched, has designed a series of reservoirs to replace alpine lakes that would be eliminated by the mine.

The impasse has highlighted Humala's struggle to neutralize Peru's polarized political environment. Some protesters have accused Humala of having moved too far to the right and being too nice to big business.

Humala has urged dialogue but a week ago the government asked Newmont to temporarily halt work on the Conga project after the protests turned violent.