Fresh clashes erupted between police and protesters in the southern Indian state of Tamil Nadu on Wednesday, taking the death toll among demonstrators against a major mining operation up to 12.

Police opened fire on a planned protest on Tuesday against the expansion of a copper smelter which, activists said, was already polluting air and water for local residents and fisheries.

Officials said 11 were killed in those clashes, and another person died on Wednesday when a demonstration outside a government hospital treating the protesters also descended into violence. At least a dozen were wounded across the two days.

People shout slogans outside Tamil Nadu House during a protest, after at least 11 people were killed when police fired on protesters (ANI via Reuters)

Videos showed fires had been started amid running clashes between authorities and protesters. In some, slumped and wounded protesters could be seen being hauled off the streets.

The alleged brutality of the police crackdown has become a national political talking point, with opposition Congress leader Rahul Gandhi blaming “the bullets of the [Hindu nationalist] RSS and [prime minister Narendra] Modi”.

In the wake of the violence, the high court in the state capital Chennai ordered Sterlite Copper, a mining subsidiary of the UK-based Vedanta Resources Plc, to halt its proposed expansion in Thoothukudi.

People throw stones during the protest (ANI via Reuters)

A bus on fire during a protest against the construction of a copper smelter by Vedanta Resources, in Thoothukudi (ANI via Reuters)

"Vedanta shall cease construction and all other activities on-site proposed Unit-II of the Copper Smelting Plant at Tuticorin (Thoothukudi) with immediate effect," the order said.

But the court order represents only a temporary stay of action; it requires Vedanta to carry out a public consultation over its plan to double the smelter’s capacity, and for the local agency to make a new hearing on environmental impacts public.

The plant, one of India's biggest, had already been shut for more than 50 days and will remain closed until at least 6 June because the local pollution regulator has said the facility is not complying with environmental rules.

"Truth and reasoning have prevailed," said Fatima Babu, who had petitioned the court.

Analysts said the deaths of protesters raised questions about who was in command of the police response to Tuesday’s large scale demonstration, which was long-planned as the 100th day of an anti-Sterlite sit-in.

The state government in Tamil Nadu is run by the regional AIADMK party, a likely ally of Prime Minister Modi’s ruling BJP party in next year’s general election.

The leader of the Tamil Nadu opposition DMK party, MK Stalin, visited injured protesters in hospital on Wednesday and declared their treatment at the hands of police “inhumane”. He demanded the permanent closure of the Sterlite plant, and called for the removal of state police chief TK Rajendiran.

For its part, the BJP accused Mr Gandhi of “milking” the situation to attack the central government. It said law and order was a matter for individual states, and offered to teach the Congress leader basic “civics”.

But Mr Gandhi was not the only one to point the finger at the “centre”. The prominent southern Indian actor Prakash Raj tweeted “shame on Tamil Nadu’s visionless… spineless government”. He added: “Couldn’t you foresee citizens’ anguish over pollution concerns OR are you busy dancing to CENTER’s tunes to hold on to power”.

Members of the public set vehicles on fire and threw stones at police as they stormed the district government headquarters and an apartment block for Vedanta employees, the police and a company official said.

Police said they opened fire to control the situation after the crowds defied curfew orders.

Vedanta says the protests were based on "false allegations".