Valsa John had been helping tribal groups displaced by coal mining companies in Pakur district in the eastern state of Jharkhand. [break]
Her younger brother told the Press Trust of India news agency that she had spoken of facing down threats from the local "mining mafia" who are alleged to force tribal people off their land.
Pakur´s police superintendent Amarnath Khanna told AFP she was hacked to death while sleeping at home late on Tuesday night.
"Nearly 50 people with traditional arms in their hands attacked her house and killed her," Khanna said, adding that no arrests had yet been made over the murder.
India´s mining sector has been plagued by reports of massive corruption and protests from impoverished local communities who receive little to no benefit from the booming industry.
In an effort to address growing anger, the government approved a draft law in September which would force coal mining firms to share profits with villagers.
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