Bolivian Deputy Interior Minister Rodolfo Illanes was beaten to death after he was kidnapped by striking mineworkers on Thursday, the government said, and up to 100 people have been arrested as authorities vowed to punish those responsible.
"At this present time, all the indications are that our deputy minister Rodolfo Illanes has been brutally and cowardly murdered," Minister of Government Carlos Romero said in broadcast comments.
He said Illanes had gone to talk to protesters earlier on Thursday in Panduro, around 160 km (100 miles) from the capital, La Paz, but was intercepted and kidnapped by striking miners.
The government was trying to recover his body, Romero said.
Illanes' assistant had escaped and was being treated in a hospital in La Paz.
Protests by miners in Bolivia demanding changes to laws turned violent this week after a highway was blockaded. Two workers were killed on Wednesday after shots were fired by police. The government said 17 police officers had been wounded.
The National Federation of Mining Cooperatives of Bolivia, once strong allies of leftist President Evo Morales, began what they said would be an indefinite protest after negotiations over mining legislation failed.
Protesters have been demanding more mining concessions with less stringent environmental rules, the right to work for private companies, and greater union representation.
The vast majority of miners in Bolivia, one of South America's poorest countries, work in cooperatives, scraping a living producing silver, tin and zinc. There are few foreign-owned mining firms, unlike in neighbouring Peru and Chile.
Natural gas accounts for roughly half of Bolivia's total exports. Ex-coca grower Morales nationalised Bolivia's resources sector after taking power in 2006, initially winning plaudits for ploughing the profits into welfare programs and boosting development.
However, his government has been dogged by accusations of cronyism and authoritarianism in recent years, and even the unions who were once his core support have soured on him as falling prices have crimped spending.