A low-caste teenage boy in India has been burnt to death for letting his goat stray into paddy fields belonging to a high-caste farmer, police say.

Sai Ram was attacked on Wednesday night in a village in Rohtas district of Bihar, one of India's most populous, underdeveloped and impoverished states.

"Ram was first badly beaten by [the] accused and later set on fire," Chandan Kushwaha, superintendent of police in Rohtas said.

"He was declared dead when brought to a local government hospital."

The 15-year-old belonged to the Mahadalit caste, the lowest in India's hereditary social system.

Mr Kushwaha said police had registered a murder case against the farmer, who was arrested on Thursday, after the boy's family went to police requesting action.

"Police have begun their investigation into the case and raids continued to arrest the accused, who had absconded after the incident," he said.

More than 138 million Indians belong to the lowest caste known as the Dalits, or "the oppressed," the term the community prefers to the "untouchables".

In Bihar, the Mahadalits have been classed by the state government as the poorest of the Dalits.

Angry members of the opposition Communist Party of India protested in the area against the attack, blocking roads, the Press Trust of India (PTI) news agency reported.

State chief minister Jitan Ram Manjhi, himself a Mahadalit, demanded swift action to bring those responsible to justice, and pledged compensation for the boy's family, the PTI said.

AFP