An Indian teenager who dared to write a love letter to a sweetheart from a higher caste was beaten and paraded through the streets before being thrown under a train and killed.

Manish Kumar, 15, a member of India’s Dalit, or “Untouchable”, community, was seized by a gang of men as he went to his village school.

He was beaten and his head was shaved before he was thrown on to the tracks, as his mother begged for mercy, witnesses said. It was alleged that police looked on as the incident took place.

A teenager has since been arrested and a policeman has been suspended. Five other men were detained.

“The accused killed the boy for writing a love letter to a girl of the same village. There was a scuffle and he was pushed under a train,” Rajesh Kumar, the superintendent of police in Kaimur, the district of the impoverished northern state of Bihar where the attack took place, told The Times.

Manish died because he was a member of the Ravidas community, a Dalit sub-caste that has been confined historically to working with leather – a profession deemed unclean by Hindus, for whom cows are sacred.

Another term by which the group is known, chamar, is considered a grave insult.

Three months ago the boy sent a letter “expressing his interest” in a girl from the Dhobi community, another Dalit sub-caste, which has traditionally washed clothes for a living – but is fractionally above the Ravidas in the Hindu hierarchy. The note was discovered by the girl’s parents.

Activists said that the murder underscored how even India’s oppressed Dalits were divided by the caste system, an ancient and rigid social structure that dictates that a person’s birth governs their profession and status. Prakash Louis, a sociologist based in Bihar, said: “The intense cruelty of this attack has captured interest, but similar incidents are occurring daily.”

Far from being a fading vestige of “old India”, experts say that the caste system continues to evolve, even as the country experiences an economic renaissance. The Dhobi community in Manish’s area has benefited from education to a greater degree than the Ravidas, and has been able to win better jobs as a result, local activists say. The imbalance has meant that discrimination against Ravidas by better-off Dalits has increased in recent years.

“Caste-based atrocities are common here: rapes, murders, beatings. The privileged prey on the weak,” said Uday Kumar, a director of the Bihar-based Dalit Association for Social and Human Rights Awareness. Similar abuses are reported regularly across India. Alamelu, the leader of a group of Dalit women in the southern state of Tamil Nadu, said: “If a Dalit labourer is a girl, it’s accepted that they will be sexually exploited. Our high-caste landlords might consider us untouchable in the daytime, but at night it’s a different matter.”

The caste system groups the dominant Hindu population into four var-nas, a term that can be translated as category or colour. The priest Brahmins at the top are followed by the warrior Kshatriyas, then the merchant or farmer Vaishyas and the artisan Sudras. The Dalits are beneath this hierarchy. The structure is complicated by the associated Jati system, which divides society into thousands of subgroups, based on occupation.

Since Independence, dozens of laws have been passed in an attempt to stamp out discrimination. However, activists say that legislation is flouted regularly, especially in India’s rural hinterlands. Positive discrimination policies, under which government jobs and university places are reserved for members of lower castes, have bred resentment and triggered violent protests among those not eligible.

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