A police officer was killed on Tuesday when violence broke out at a demonstration in western India to protest an attack on low-caste villagers by cow protection vigilantes.

Police fired teargas shells and used sticks to try to control stone-throwing crowds angered by attempts to stop them from demonstrating in western Gujarat state.

Hundreds of people have been detained as authorities try to contain the unrest, which began on Monday and has since spread.

One senior officer was fatally wounded in Tuesday’s clashes and several more suffered minor injuries, the local superintendent of police J A Patel told AFP.

The protests erupted after video footage emerged of an attack last week on four villagers from the lowest Dalit caste who were taking a dead cow to be skinned.

Cows are considered sacred by Hindus and killing them is banned in Gujarat, but the villagers said the animal had died of natural causes.

Low-caste villagers are commonly tasked with removing the corpses of dead cows from the streets of India, where the animals often roam freely.

Local lawmaker Shambhu Prasad Tundiya said the violence was a sign of frustration among low-caste Hindus after years of discrimination by people from higher castes who often went unpunished.

“Such atrocities by upper-caste people on Dalits have been happening time and again since ages. This is unacceptable,” Tundiya told reporters.

Gujarat Chief Minister Anandiben Patel meanwhile said 16 people had now been arrested over the attack and appealed for calm after a series of apparent attempted suicides among Dalits.

“I appeal to Dalit youth to not resort to desperate measures like attempting suicide & assure them that govt is with Una victims,” she tweeted, referring to the town where the original attack took place.

“Govt will make sure accused will face a speedy trial.”

The video showed the four half-naked men tied to a car as the activists took turns to thrash them with belts and batons at a crowded marketplace.

Two more Dalits were beaten up after they tried to save the other four.

Attacks by vigilante groups on cow traders and smugglers have increased since Hindu nationalist Prime Minister Narendra Modi won power in 2014.

At least five Muslim men were killed last year by Hindu mobs on suspicion of eating beef or smuggling cows across the country.